glx: Don't downgrade the visual caveat from the server

glx: Don't downgrade the visual caveat from the server

Mesa marks accumful fbconfigs as slow (for no especially good reason,
it's only accum operations that aren't accelerated, and we could fix
that). NVIDIA doesn't. All that mismatching on this attribute can do is
prevent a config from working exactly as well as it possibly can.

Trust the server's opinion here (but warn if you ask for warnings).

Acked-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1648>