Add support for RGBA format in GpuShaderCreator for better Metal support?

Hi everyone,

Only single channel and RGB texture formats are currently supported in `GpuShaderCreator::TextureType.` However, it appears that Apple does not support simple RGB format textures in Metal. Please refer to https://developer.apple.com/documentation/metal/mtlpixelformat.

Working with OCIO shaders in Metal becomes inconvenient. When I query the GpuShaderDesc for a Metal shader and its textures, I sometimes get RGB textures that are not accepted by Metal. I have to insert a placeholder alpha channel to convert them to RGBA format.

Is it an intentional decision that OCIO does not support RGBA texture? If not, is it a plan for the future?

Thanks a lot!