MaterialX: fix for GLSL shader generation if not using bindless textures

[pmolodo]

Description of Change(s)

Currently, if you try to use usdview to view an object with a materialX shader applied, and bindless textures are not enabled, then you will get an error:

error C7554: OpenGL requires sampler variables to be explicitly declared as uniform

This can be reproduced on the dev branch by building a stock USD, then setting GLF_ENABLE_BINDLESS_SHADOW_TEXTURES=0 in your environment before launching usdview on any file displaying materialx shaders.

Fixes Issue(s)

• Ability to view MaterialX in OpenGL if not using bindless textures.

[pmolodo]

This is a bit messy, as it adds more special case branching. Wasn't sure how to do a cleaner version that didn't require much more intrusive changes, though...

[pmolodo]

Attaching a simple scene that can be used to show the issue - thanks to Jerry Gamache and Spiff (https://groups.google.com/g/usd-interest/c/8Wy8zi1SN-o/m/YS4P1TFPCAAJ)

Just unzip and do:

Windows:

set GLF_ENABLE_BINDLESS_SHADOW_TEXTURES=0
usdview jerry_gamache_mtlx_example.usda

Linux/MacOS:

export GLF_ENABLE_BINDLESS_SHADOW_TEXTURES=0
usdview jerry_gamache_mtlx_example.usda

jerry_gamache_mtlx_example.zip

[jilliene]

Filed as internal issue #USD-6939

[klucknav]

Thank you for the change. Unfortunately, we cannot directly merge your change as there are a handful of changes we would like to make. But we have a fix for the underlying issue based on your change that should be in dev soon. Thanks again.

[pmolodo]

Great! Was by no means enamored of my particular solution - as long as the underlying issue gets fixed!