egl/glx: add fallback for zink loading

if the driver attemping to load is not zink and not software, then
attempt a zink fallback on failure

this conservatively handles the case of "only zink is built", though it
is going to be noticeably slower at startup than loading zink directly

Reviewed-by: Adam Jackson <ajax@redhat.com>
220 jobs for !16168 with kopper-fallback in 48 minutes and 21 seconds (queued for 59 seconds)
merge request