xwayland: Do flush GPU work in xwl_present_flush

The Present code sends the idle notification event to the client after
xwl_present_flush returns. If we don't flush our GPU work here, the
client may race to draw another frame to the same buffer, so we may end
up copying (parts of) that new frame instead of the one we meant to.

Fixes xorg/xserver#835

Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
3 jobs for xwl_present_flush-glamor_block_handler in 5 minutes and 16 seconds (queued for 4 seconds)