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 master in 5 minutes and 7 seconds
Status Job ID Name Coverage
  Docker Image
passed #1602532
debian-testing

00:00:22

 
  Build And Test
passed #1602533
autotools

00:04:45

passed #1602534
meson

00:02:04

passed #1602535
meson-noglamor

00:02:12