Fix geomPropValue for GLSL to output varying vs unifom

[bernardkwok]

Currently the custom GLSL code generation node (MaterialXGenGlsl\Nodes\GeomPropValueNodeGlsl.cpp)
is outputting a uniform instead of a varying parameter. This should be changed to output a varying to match
the current specification (as of v1.38).

Work:

• Fixes to GeomPropValueNodeGLSL.cpp
• Testing for raw glsl, ogsfx, and ogsxml variants.

Follow-up:

• Checking that this will still work with SPIR-V cross-compilation.

---

Whether to extend geomPropValue to indicate that the input is a uniform is a separate issue beyond the scope of this fix.
A placeholder item can be found here: #1051.

[JGamache-autodesk]

Testing raises some questions about handling invalid types in GLSL. What should be the GLSL code output for types bool and string?
Also there is a requirement to add the keyword flat to integer varying inputs. Easy to implement, but might trip the HLSL converter.

[bernardkwok]

• For the first point, as you can't have boolean or string vertex streams we could leave these as uniforms. We are no worse off than before. Any language which does not support "string" we output integers instead and leave it up the the binding process to determine the string->integer mapping.
• As for integer streams, I'm not sure of the frequency of these so maybe we could also defer this for now and also just keep them as uniforms.
• Which leaves us with 2-4 channel float support.

[bernardkwok]

Pulling out a note from PR as there was a possible concern with adding in the "flat" qualifier for integer varyings. Seems this is a non-issue (See KhronosGroup/glslang#940)