Refine MaterialX render module build configuration (#1970)

Add more refined control over building MaterialX render modules. Specifically USD only needs the core render module in some build configurations (iOS).

Also add a guard to ensure that MaterialXGenGlsl is built if MaterialXGenMsl is built, because of the dependency in the data library.  USD uses `MaterialX::GlslShaderGenerator::TARGET` to locate those resources.

CMakeLists.txt

@@ -43,6 +43,7 @@ option(MATERIALX_BUILD_GEN_OSL "Build the OSL shader generator back-end." ON)
 option(MATERIALX_BUILD_GEN_MDL "Build the MDL shader generator back-end." ON)
 option(MATERIALX_BUILD_GEN_MSL "Build the MSL shader generator back-end." ON)
 option(MATERIALX_BUILD_RENDER "Build the MaterialX Render modules." ON)
+option(MATERIALX_BUILD_RENDER_PLATFORMS "Build platform-specific render modules for each shader generator." ON)
 option(MATERIALX_BUILD_OIIO "Build OpenImageIO support for MaterialXRender." OFF)
 option(MATERIALX_BUILD_TESTS "Build unit tests." OFF)
 option(MATERIALX_BUILD_BENCHMARK_TESTS "Build benchmark tests." OFF)
@@ -104,6 +105,11 @@ if (MATERIALX_BUILD_JS)
     set(MATERIALX_BUILD_TESTS OFF)
 endif()
 
+if(MATERIALX_BUILD_GEN_MSL)
+    # All hardware shading languages currently depend on the GLSL shader generator.
+    set(MATERIALX_BUILD_GEN_GLSL ON)
+endif()
+
 set(MATERIALX_PYTHON_VERSION "" CACHE STRING
     "Python version to be used in building the MaterialX Python package (e.g. '3.9').")
 set(MATERIALX_PYTHON_EXECUTABLE "" CACHE FILEPATH
@@ -160,6 +166,7 @@ mark_as_advanced(MATERIALX_BUILD_GEN_OSL)
 mark_as_advanced(MATERIALX_BUILD_GEN_MDL)
 mark_as_advanced(MATERIALX_BUILD_GEN_MSL)
 mark_as_advanced(MATERIALX_BUILD_RENDER)
+mark_as_advanced(MATERIALX_BUILD_RENDER_PLATFORMS)
 mark_as_advanced(MATERIALX_BUILD_OIIO)
 mark_as_advanced(MATERIALX_BUILD_BENCHMARK_TESTS)
 mark_as_advanced(MATERIALX_BUILD_SHARED_LIBS)
@@ -465,15 +472,22 @@ endif()
 # Add rendering and viewer subdirectories
 if(MATERIALX_BUILD_RENDER)
     add_subdirectory(source/MaterialXRender)
-    add_subdirectory(source/MaterialXRenderHw)
-    if (MATERIALX_BUILD_GEN_GLSL)
-        add_subdirectory(source/MaterialXRenderGlsl)
-    endif()
-    if (APPLE AND MATERIALX_BUILD_GEN_MSL)
-        add_subdirectory(source/MaterialXRenderMsl)
-    endif()
-    if (MATERIALX_BUILD_GEN_OSL)
-        add_subdirectory(source/MaterialXRenderOsl)
+    if(MATERIALX_BUILD_RENDER_PLATFORMS)
+        set(MATERIALX_BUILD_RENDER_HW OFF)
+        if(MATERIALX_BUILD_GEN_GLSL AND NOT MATERIALX_BUILD_APPLE_EMBEDDED)
+            set(MATERIALX_BUILD_RENDER_HW ON)
+            add_subdirectory(source/MaterialXRenderGlsl)
+        endif()
+        if(MATERIALX_BUILD_GEN_MSL AND APPLE)
+            set(MATERIALX_BUILD_RENDER_HW ON)
+            add_subdirectory(source/MaterialXRenderMsl)
+        endif()
+        if(MATERIALX_BUILD_RENDER_HW)
+            add_subdirectory(source/MaterialXRenderHw)
+        endif()
+        if(MATERIALX_BUILD_GEN_OSL)
+            add_subdirectory(source/MaterialXRenderOsl)
+        endif()
     endif()
     if(MATERIALX_BUILD_VIEWER)
         add_subdirectory(source/MaterialXView)