An attempt to resolve T83586

Cycles currently only allows 1 to 4 channel image to be loaded, if greater like with a exr image with rgba and z it will generate a pink texture instead.
Simply changing to a max 5 channel is not a long term solution. However, some functions afterward the condition will trim of the excess channels anyway.

Currently it seems that only BlenderImageLoader::load_pixels might be affected by it since it uses the computed size based on the channels variable.