compositor: Generalize output previous damage into per buffer damage

compositor: Generalize output previous damage into per buffer damage

This is a more generic fix for the issue solved in 4f521731 where
damage obscured by overlays could be lost in one of the output buffers
due to rapid move of a surface in an overlay plane.

This changes the renderer so it keeps track of the damage in each
buffer. Every time a new frame is drawn, the damage of the frame is
added to all the buffers and the rendered regions are cleared from
the current buffer's damage.