Add XYZ-to-working-space transform for hardware shaders

[r800360]

Fixes #2859

Summary

• Adds GenOptions::xyzToWorkingSpace, defaulting to the CIE XYZ to lin_rec709 transform.
• Exposes the transform as the private hardware-shader uniform u_xyzToWorkingSpace.
• Uses the uniform for blackbody emission and thin-film iridescence.
• Removes the two hardcoded XYZ-to-RGB matrices.
• Adds a focused GLSL shader-generation test covering the default transform, custom overrides, private-uniform propagation, token substitution, and both shader consumers.

Testing

• MaterialXTest "GenShader: GLSL XYZ to Working Space"
• Full MaterialXTest suite
• 65 test cases and 14,579 assertions passed

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: r800360 / name: Rohan Sachdeva (803f959)