This allows code outside of the render pipeline to make proper
decisions about how the imbuf of render passes are to be handled.
For example, IMB_create_gpu_texture() will now properly select
single channel grayscale texture format for depth pass coming from
multilayer EXR, additionally solving assert in the GPU compositor
code which verifies expected and actual imbuf texture format.
Set the number of planes based on the number of pass channels.
If the pass contains 2 passes or more than 4 passes set the number
of planes to the previously used value of 32.
This is needed because quite some areas check for the number of
planes for various optimizations. For example, this is one of the
factors which make IMB_create_gpu_texture() to choose the texture
format. If the number of planes for the depth pass is set to the
previously used this function will never consider using single
channel GPU texture.
Unfortunately, this change is not enough to make the GPU texture
to use single channel format as the color space of the image
buffer is also checked, and that is nullptr which means scene linear.
Sergey Sharybin