Merge (#47)

* Add enumeration support to web viewer (AcademySoftwareFoundation#1632)

- Scan for `enum` and `enumvalues` if attributes exist on input. If no `enumvalues` exist then map to "default" of 0..<enum list size>.
- Create a drop-down for each enumerated input.

* Initial nprlib with viewdirection node (AcademySoftwareFoundation#1631)

This changelist introduces an NPR (non-photorealistic rendering) data library to MaterialX, initially consisting of a single `viewdirection` node.

The NPR data library is designed for the portable expression of artistic, non-physically-based materials, with common examples being cartoon shading, architectural diagrams, and edge highlighting.  In future versions of MaterialX, we expect this data library to be extended with additional primitives and artist-facing graphs as they are requested by the graphics community and approved by the MaterialX TSC.

For shader generation, MaterialX initially supports the NPR data library in GLSL, ESSL, MSL, and OSL, with MDL being omitted for now, based on intentional restrictions in its design philosophy.

* Improve shader generation for viewdirection

This changelist improves shader generation logic for the new viewdirection node, allowing it to work correctly in derived hardware languages such as ESSL.

* Add creatematrix nodes to build matrices from vectors (AcademySoftwareFoundation#1553)

I'm opening this PR to add creatematrix, a constructor for Matrix33 from 3 Vector3s, Matrix44 from 4 Vector3s and Matrix44 from 4 Vector4s from the specification document (https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/documents/Specification/MaterialX.Specification.md#math-nodes).

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Co-authored-by: Jonathan Stone <[email protected]>
Co-authored-by: Frieder Erdmann <[email protected]>