When drawing entirely opaque regions, we traditionally kept blending on
simply because it made the code more convenient and obvious to handle.
However, this can cause lots of performance issues on GPUs that aren't
too powerful, as they have to readback the buffer underneath.
Keep track of the opaque region set by windows (through _NET_WM_OPAQUE_REGION,
standard RGB32 frame masks or similar), and draw those rectangles
separately through a different path with blending turned off.
https://bugzilla.gnome.org/show_bug.cgi?id=706930
ab4c929a07