USD Viewer: render state cache errors on MacOS/Metal

[TheMostDiligent]

Diligent Engine: ERROR in ParseMSL() (ShaderMtlImpl.mm, 377): Failed to load function 'main' in shader 'Full-screen Triangle VS'.

Diligent Engine: ERROR in CreateDeviceObject() (RenderDeviceBase.hpp, 385): Failed to create Shader object 'Full-screen Triangle VS'

Diligent Engine: ERROR in SerializeShader() (RenderStateCache.cpp, 911): Failed to serialize shader 'Full-screen Triangle VS'.