This allows compositors to only process damaged (that is, updated)
regions of our window buffer, which for larger window sizes (think 4k)
should significantly reduce compositing workload under compositors that
support/honor it, which is good for performance, battery life and lower
latency over remote connections like VNC.
On Wayland, clients are expected to always report correct damage, so
this makes us a good citizen there. It can also aid remote desktop
(waypipe, rdp, vnc, ...) and other types of screencopy by having damage
bubble up correctly.
Fixes#3186.
8f1abe13e6