When taking the scanout path we still want to clear the
redraw-clip from the stage-view in order to ensure we skip
frames in `handle_frame_clock_frame()` if no new redraw-clip
was recorded.
This was not done previously as the accumulated redraw-clip was
needed for the next repaint, likely under the assumption that
scheduling a scanout repeatedly would be computationally cost-free.
This assumption does not hold in a VRR world.
In order to archive both, an accumulated redraw-clip for the next
paint and frame-skipping during scanout, introduce new API to defer
and accumulate redraw-clips until the next repaint.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2480>
550f09a5e7