present/scmd: Check that the flip and screen pixmap pitches match

If they don't, flipping will result in corrupted display.

Test case:

* Run Xorg at 1920x1080 with no window manager
* glxgears -geometry 2048x1080
3 jobs for present_scmd-check_flip-pitch in 8 minutes and 29 seconds (queued for 115 minutes and 12 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #174990

00:00:30

 
  Build And Test
passed autotools-build-and-test #174991

00:06:28

passed meson-build-and-test #174992

00:01:30