On GLES2 reading and writing some Cogl formats is not supported
natively. In those cases we use another format to do the reading and
writing. When the internal format and the temporary format differ in
premultiplication, Cogl tries to adjust for it.
Opaque Cogl formats don't have the premult bit set but our internal
format is a premult format. Cogl tries to adjust for it but completely
misses that the opaque format doesn't have an alpha channel and it
should not do so at all.
So skip the premult adjusting when the Cogl format has no alpha channel.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3065>
5696d61a67