Improve specification of geometric spaces (#2151)

- Centralize the list of supported geometric spaces in a new `Geometric Spaces` section.
- Remove `tangent` and `common` from the list of geometric spaces, as they are not supported in the codebase, and more design discussion would be needed to propose their addition.
- Remove the `space` input from `heighttonormal`, as it's not supported in the codebase, and more design discussion would be needed to propose its addition.

documents/Specification/MaterialX.Specification.md

@@ -56,6 +56,7 @@ This document describes the core MaterialX specification.  Companion documents [
   [Noise Nodes](#noise-nodes)  
   [Shape Nodes](#shape-nodes)  
   [Geometric Nodes](#geometric-nodes)  
+   [Geometric Spaces](#geometric-spaces)  
   [Application Nodes](#application-nodes)  
 
  [Standard Operator Nodes](#standard-operator-nodes)  
@@ -1050,23 +1051,23 @@ Standard Geometric nodes:
 <a id="node-position"> </a>
 
 * **`position`**: the coordinates associated with the currently-processed data, as defined in a specific coordinate space.  This node must be of type vector3.
-    * `space` (uniform string): the name of the coordinate space in which the position is defined.  Default is "object", see below for details.
+    * `space` (uniform string): the name of the coordinate space in which the position is defined.  Default is "object", see [Geometric Spaces](#geometric-spaces) for supported options.
 
 <a id="node-normal"> </a>
 
 * **`normal`**: the geometric normal associated with the currently-processed data, as defined in a specific coordinate space.  This node must be of type vector3.
-    * `space` (uniform string): the name of the coordinate space in which the normal vector is defined.  Default is "object", see below for details.
+    * `space` (uniform string): the name of the coordinate space in which the normal vector is defined.  Default is "object", see [Geometric Spaces](#geometric-spaces) for supported options.
 
 <a id="node-tangent"> </a>
 
 * **`tangent`**: the geometric tangent vector associated with the currently-processed data, as defined in a specific coordinate space.  This node must be of type vector3.
-    * `space` (uniform string): the name of the coordinate space in which the tangent vector is defined.  Default is "object", see below for details.
+    * `space` (uniform string): the name of the coordinate space in which the tangent vector is defined.  Default is "object", see [Geometric Spaces](#geometric-spaces) for supported options.
     * `index` (uniform integer): the index of the texture coordinates against which the tangent is computed.  The default index is 0.
 
 <a id="node-bitangent"> </a>
 
 * **`bitangent`**: the geometric bitangent vector associated with the currently-processed data, as defined in a specific coordinate space.  This node must be of type vector3.
-    * `space` (uniform string): the name of the coordinate space in which the bitangent vector is defined.  Default is "object", see below for details.
+    * `space` (uniform string): the name of the coordinate space in which the bitangent vector is defined.  Default is "object", see [Geometric Spaces](#geometric-spaces) for supported options.
     * `index` (uniform integer): the index of the texture coordinates against which the tangent is computed.  The default index is 0.
 
 <a id="node-bump"> </a>
@@ -1102,17 +1103,13 @@ Standard Geometric nodes:
 Additionally, the `geomcolor` and `geompropvalue` nodes for color3/color4-type properties can take a `colorspace` attribute to declare what colorspace the color property value is in; the default is "none" for no colorspace declaration (and hence no colorspace conversion).
 
 
-<a id="space-values"> </a>
+#### Geometric Spaces
 
 The following values are supported by the `space` inputs of Geometric nodes and when transforming from one space to another:
 
-
 * "model": The local coordinate space of the geometry, before any local deformations or global transforms have been applied.
 * "object": The local coordinate space of the geometry, after local deformations have been applied, but before any global transforms.
 * "world": The global coordinate space of the geometry, after local deformations and global transforms have been applied.
-* "tangent": A coordinate space defined by the tangent, bitangent and normal vectors of the geometry.
-
-Applications may also reference other renderer-specific named spaces, at the expense of portability.
 
 
 
@@ -1343,21 +1340,21 @@ Math nodes have one or two spatially-varying inputs, and are used to perform a m
 
 * **`transformpoint`**: transform the incoming vector3 coordinate from one specified space to another; cannot be used on any other stream type.
     * `in` (vector3): the input coordinate vector.
-    * `fromspace` (uniform string): the name of a vector space understood by the rendering target to transform the `in` point from; may be empty to specify the renderer's working or "common" space.
+    * `fromspace` (uniform string): the name of a vector space understood by the rendering target to transform the `in` point from; see [Geometric Spaces](#geometric-spaces) for supported options.
     * `tospace` (uniform string): the name of a vector space understood by the rendering target for the space to transform the `in` point to.
 
 <a id="node-transformvector"> </a>
 
 * **`transformvector`**: transform the incoming vector3 vector from one specified space to another; cannot be used on any other stream type.
     * `in` (vector3): the input vector.
-    * `fromspace` (uniform string): the name of a vector space understood by the rendering target to transform the `in` point from; may be empty to specify the renderer's working or "common" space.
+    * `fromspace` (uniform string): the name of a vector space understood by the rendering target to transform the `in` point from; see [Geometric Spaces](#geometric-spaces) for supported options.
     * `tospace` (uniform string): the name of a vector space understood by the rendering target for the space to transform the `in` point to.
 
 <a id="node-transformnormal"> </a>
 
 * **`transformnormal`**: transform the incoming vector3 normal from one specified space to another; cannot be used on any other stream type.
     * `in` (vector3): the input normal vector; default is (0,0,1).
-    * `fromspace` (uniform string): the name of a vector space understood by the rendering target to transform the `in` point from; may be empty to specify the renderer's working or "common" space.
+    * `fromspace` (uniform string): the name of a vector space understood by the rendering target to transform the `in` point from; see [Geometric Spaces](#geometric-spaces) for supported options.
     * `tospace` (uniform string): the name of a vector space understood by the rendering target for the space to transform the `in` point to.
 
 <a id="node-transformmatrix"> </a>
@@ -1782,7 +1779,6 @@ Convolution nodes have one input named "in", and apply a defined convolution fun
 * **`heighttonormal`**: convert a scalar height map to a normal map of type vector3.
     * `in` (float): the input value or nodename
     * `scale` (float): the scale of normal map deflections relative to the gradient of the height map.  Default is 1.0.
-    * `space` (string): the space in which the output normal map vector should be; defaults to "tangent".