sourceColorSpace not set/exposed (USDU-242)

[Scraft]

At the moment, an exported USD, with linear textures, when opened in Blender, will instead read the textures as sRGB. This can be fixed by setting sourceColorSpace for the shader/texture.

https://graphics.pixar.com/usd/release/spec_usdpreviewsurface.html

Flag indicating the color space in which the source texture is encoded. Possible Values:
raw : Use texture data as it was read from the texture and do not mark it as using a specific color space.
sRGB : Mark texture as sRGB when reading.
auto : Check for gamma/color space metadata in the texture file itself; if metadata is indicative of sRGB, mark texture as sRGB . If no relevant metadata is found, mark texture as sRGB if it is either 8-bit and has 3 channels or if it is 8-bit and has 4 channels. Otherwise, do not mark texture as sRGB and use texture data as it was read from the texture.

I have confirmed if I manually add this to the relevant textures (in particular things like normal maps, gloss maps) it fixes rendering issues within Blender. I can see this flag is exposed in the USD SDK:

https://github.com/PixarAnimationStudios/USD/blob/faed18ce62c8736b02413635b584a2f637156bad/pxr/imaging/hdSt/udimTextureObject.cpp

Actions:

1. Expose sourceColorSpace so it can be set in code
2. Automatically set sourceColorSpace appropriately during export.