egl: mark the rest of the callbacks as mandatory or optional

Suggested-by: Frank Binns <frank.binns@imgtec.com>
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5861>
1 file changed