This reverts commit 4918893326.
This commit prevented cogl_stage_cogl_redraw_view() from skipping
swap buffers entirely if the invalidation region ended up empty.
This meant we were actually swapping buffers when we didn't need to.
The source of the glitches was fixed more properly, so this just adds
extra work.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/898
6bfa61a3a7