Fix cmake rebuild for MaterialXRenderGlsl. Fix PyTinyObjectLoader.load.

[Javantea]

You need to take care about adding files from now on but that is normal. Fix PyTinyObjLoader.load so that it returns meshes instead of not returning meshes. These are the first patches toward getting Python rendering working which should be a lot of fun.

[linux-foundation-easycla[bot]]

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• :white_check_mark: login: Javantea / name: Javantea (24be91e45de55bea0d9084cd20bcdc74495d2f1c)

[jstone-lucasfilm]

Thanks for taking this work forward, @Javantea and @kwokcb, and indeed the idea of MaterialX rendering from Python is exciting!