present: Check that 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

The Present extension code in xserver 1.21 will check for this.

Tested-by: Jax Lin <jax.lin@amd.com>