During rendering, restack offloaded subsurfaces below the main
surface, and clear the area so they peek through. After rendering,
raise the last subsurface if we haven't drawn over it.
If the rendering operation is over an opaque region, we can potentially
avoid clearing a large section of the framebuffer destination. Some cases
you do want to clear, such as when clearing the whole contents as some
drivers have fast paths for that to avoid bringing data back into the
framebuffer.
