compositor: Accumulate damage per plane

compositor: Accumulate damage per plane

When we analyze and accumulate damage prior to repainting, we need to
accumulate damage per plane, so that whatever damage a surface
contributes is accumulated in the plane that it's assigned to.  Before,
we would always accumulate damge in the primary plane, which caused
repaints in the primary plane whenever a surface in a sprite or
framebuffer was damaged.  Eliminating this repaint is a big win for
cases where we pageflip to a client surface or use a sprite overlay.

This also prepares for fixing the missing cursor updates, since we
now track damage to the cursor surface in a dedicated sprite plane.