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
195 jobs for !16168 with kopper-fallback in 36 minutes and 18 seconds (queued for 19 seconds)
merge request