This adds the processing required to import and export simple material graphs utilizing the UsdUVTexture outputs channels.
If only r, g or b are specified as output, we hook up a `Separate Color` node and connect the appropriate channel from there.
(if a is specified as output, the Alpha output of an image texture node was used already)
On the export side, we traverse from the socket to the image texture node, and if a `Separate Color` on the way, we are using the channel from there to put on the output.
https://openusd.org/release/spec_usdpreviewsurface.html#texture-reader
Pull Request: https://projects.blender.org/blender/blender/pulls/117901
228ee8f1c3