Refine MaterialX render module build configuration

[ld-kerley]

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.

[jstone-lucasfilm]

@ld-kerley I like the idea behind this proposal, though the split between MATERIALX_BUILD_RENDER and MATERIALX_BUILD_RENDER_CORE seems counterintuitive to me, as there's no corresponding module named MaterialXRenderCore.

As an alternative, what if we added a MATERIALX_BUILD_RENDER_PLATFORMS flag, defaulting to ON, with all platform-specific render modules gated by this flag?

This would allow USD developers to opt out of platform-specific render modules such as MaterialXRenderGlsl and MaterialXRenderOsl, without losing access to the MaterialXRender module that they still need.

[jstone-lucasfilm]

Thanks for the updates, @ld-kerley, and this looks really promising! I'll take a closer look in upcoming days, and let's see if we can get this change ready to include in MaterialX v1.39.2.

[jstone-lucasfilm]

This looks good to me, thanks @ld-kerley!