mesa: Add support for array formats of depth and stencil.

mesa: Add support for array formats of depth and stencil.

In desktop GL, you can specify things like GL_DEPTH_COMPONENT/GL_BYTE as a
ReadPixels format, and we need to be able to represent that to see if we
have proper MESA_FORMATs for them.  That's exactly what the
mesa_array_format enum is for.

v2: Drop _mesa from static fn.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>