memoryformat: Add gdk_memory_format_get_depth()

memoryformat: Add gdk_memory_format_get_depth()

Replace gdk_memory_format_prefers_high_depth with the more generic
gdk_memory_format_get_depth() that returns the depth of the individual
channels.

Also make the GL renderer use that to pick the generic F16 format
instead of immediately going for F32 when uploading textures.