ND_UsdPrimvarReader_vector2 fails shader generation for Hardware shading languages

[ld-kerley]

USD uses UsdPrimvarReader to access primvars from the scene - most commonly texture coordinates to feed UsdUVTexture.

The current MaterialX shader generation for the hardware shading languages (which OpenUSDs Storm uses), currently fails code generation for these nodes.

test_file.zip (updated to include texture file)

The test files zip contains three files representing the same Material.

graph LR                                                                                                                                                                                                                                                                    
    Primvar["UsdPrimvarReader"]                                                                                                                                                                                                                               
    Tex["UsdUVTexture"]                                                                                                                                                                                                                                           
    SR["UsdPreviewSurface"]                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                            
    Primvar --> Tex                                                                                                                                                                                                                                                   
    Tex --> SR                                                                                                                                                                                                                                              

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test_uvtexture_uv_primvarreader.usda

Which is the USDShade version - this renders fine in Strom

test_uvtexture_uv_primvarreader_mx.usda

Which is the MaterialX version in USD - this does not render in Storm.

And emits the following error...

ERROR: Usdview encountered an error while rendering.
	Error in 'pxrInternal_v0_26_5__pxrReserved__::HdSt_ApplyMaterialXFilter' at line 1629 in file /Users/leekerley/dev/github_ld_kerley/OpenUSD/pxr/imaging/hdSt/materialXFilter.cpp : 'Unable to create the Glslfx Shader.
MxException: No 'geomprop' parameter found on geompropvalue node 'primvar'. Don't know what property to bind'
Coding Error: in HdSt_ApplyMaterialXFilter at line 1629 of /Users/leekerley/dev/github_ld_kerley/OpenUSD/pxr/imaging/hdSt/materialXFilter.cpp -- Unable to create the Glslfx Shader.
MxException: No 'geomprop' parameter found on geompropvalue node 'primvar'. Don't know what property to bind

test_uvtexture_uv_primvarreader.mtlx

A pure MaterialX version of the same material structure - note some minor tweaks were needed to address other differences noted in other issues between MaterialX and USD. This does not render in MaterialXView