UsdPrimvarReader incompatible node types

[ld-kerley]

The USDPreviewSpecification lists a number of different supported UsdPrimvarReader types, the MaterialX implementations do not match this list one-to-one. There are some challenges here with the differing availability of types between the two systems.

Currently we have

USD	MaterialX	Status
UsdPrimvarReader_float	ND_UsdPrimvarReader_float	✅
UsdPrimvarReader_float2	ND_UsdPrimvarReader_vector2	⚠️
UsdPrimvarReader_float3	ND_UsdPrimvarReader_vector3	⚠️
UsdPrimvarReader_float4	ND_UsdPrimvarReader_vector4	⚠️
UsdPrimvarReader_int	ND_UsdPrimvarReader_integer	⚠️
UsdPrimvarReader_string	ND_UsdPrimvarReader_string	✅
UsdPrimvarReader_normal	ND_UsdPrimvarReader_vector3	⚠️
UsdPrimvarReader_point	ND_UsdPrimvarReader_vector3	⚠️
UsdPrimvarReader_vector	ND_UsdPrimvarReader_vector3	⚠️
UsdPrimvarReader_matrix	Does not exist	❌
Does not exist	ND_UsdPrimvarReader_boolean	❌
Does not exist	ND_UsdPrimvarReader_filename	❌

We clearly need to add a matrix primvar reader to the MaterialX implementation.

I think we should consider removing the boolean and filename nodes, unless we can investigate adding them to USD. We want to be able to allow people to 1:1 round trip their data back and forth without loss.

For the other ⚠️ cases above, one possible solution is to align these names perfectly, creating aliases for the normal/point/vector USD cases.

We could end up with

USD	MaterialX	Status
UsdPrimvarReader_float	ND_UsdPrimvarReader_float	✅
UsdPrimvarReader_float2	ND_UsdPrimvarReader_vector2 -> ND_UsdPrimvarReader_float2	✅
UsdPrimvarReader_float3	ND_UsdPrimvarReader_vector3 -> ND_UsdPrimvarReader_float3	✅
UsdPrimvarReader_float4	ND_UsdPrimvarReader_vector4 -> ND_UsdPrimvarReader_float4	✅
UsdPrimvarReader_int	ND_UsdPrimvarReader_int	✅
UsdPrimvarReader_string	ND_UsdPrimvarReader_string	✅
UsdPrimvarReader_normal	ND_UsdPrimvarReader_normal (aliased to ND_UsdPrimvarReader_float3)	✅
UsdPrimvarReader_point	ND_UsdPrimvarReader_point (aliased to ND_UsdPrimvarReader_float3)	✅
UsdPrimvarReader_vector	ND_UsdPrimvarReader_vector (aliased to ND_UsdPrimvarReader_float3)	✅
UsdPrimvarReader_matrix	ND_UsdPrimvarReader_matrix	✅

Where we're renaming the MaterialX ND_UsdPrimvarReader_vectorX to ND_UsdPrimvarReader_floatX, and ND_UsdPrimvarReader_normal, ND_UsdPrimvarReader_point and ND_UsdPrimvarReader_vector all alias to ND_UsdPrimvarReader_float3.