Use XSyncSetPriority()

Use XSyncSetPriority() to prioritize the compositor above applications
for X server priority. In practice, this makes little difference because
the Xorg "smart scheduler" will schedule in a single application for
time slices that exceed the frame drawing time, but it's theoretically
right and might make a difference if the X server scheduler is improved.

https://bugzilla.gnome.org/show_bug.cgi?id=685463