ClutterActor: Preserve valid paint volumes till the next relayout/repaint
2017-04-29: Carlos says:
Cuts down approximately 55% of paint volumes calculation when there's windows that redraw frequently, but don't move.
2018-04-20: Daniel says:
I have tested this patch and it cuts down up to 70% of paint volume calculations for me. That equates to a relative reduction in gnome-shell's CPU usage by 10-20%.