present: pixmap pitch tweaks in check_flip

We need to check the pitch again even with upcoming xserver releases, but only with older kernel drivers.

Flipping to a buffer with different pitch can happen e.g. when there's a window which is wider than the root window, and fully covers it.

Patch 3 takes advantage of https://patchwork.freedesktop.org/patch/320425/.

/cc @kazlaus