xwayland/present: Skip queued flip when a new one becomes ready
If multiple flips become ready for the same MSC, we would previously execute them all sequentially, one per MSC for sync flips. This could result in an unbounded flip queue and corresponding memory consumption. With implicit sync, leave the mailbox handling to the compositor for async flips though. v2: * Use present_vblank_rec::sync_flip. Closes: xorg/xserver#1664 Fixes: e1f16fb1 ("xwayland: don't scrap pending present requests") (cherry picked from commit 0d9a54aa) Part-of: <xorg/xserver!1494>