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
(Cherry picked from commit ef91da27)
3 jobs for server-1.20-branch in 6 minutes and 45 seconds (queued for 1 second)
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debian-testing

00:00:22

 
  Build And Test
passed #191754
autotools-build-and-test

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passed #191755
meson-build-and-test

00:01:35