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)