xwayland: Call glamor_block_handler from xwl_screen_post_damage

In between the two phases introduced by the previous change. This makes
sure all pending drawing to the new buffers is flushed before they're
committed to the Wayland server.
4 jobs for xwl_screen_post_damage-two-phases in 4 minutes and 15 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Docker Image
passed #1602456
debian-testing

00:00:21

 
  Build And Test
passed #1602457
autotools

00:03:54

passed #1602458
meson

00:01:20

passed #1602459
meson-noglamor

00:00:28