MaterialX image nodes should use the "auto" color space by default

[AdrienHerubel]

When rendering 8-bit files without explicit color space info, we should use the "auto" color space to ensure better defaults.

UDIMs.zip

We should also make sure that when explicit color space info is set on the file, it is also passed to the generated image shader.

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-13415