gallium/util: add comment on util_is_format_compatible()
gallium/util: add comment on util_is_format_compatible()

From reading the code, it's not obvious what is src/dest compatible.
The list of a->b copy-compatible formats comes from Jose's original
check-in message, with some format name updates.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>
Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler