X11 backend: Retry enabling compositing when it was disabled a while ago
This removes the "OpenGLIsUnsafe" flag and replaces it with a timestamp that we can check to see how long ago we tried enabling compositing, so we can retry it if a certain amount of time has elapsed.
The main discussion point is how long we should wait before trying again. Currently I used 1 minute so there's a reasonable chance that we'd try again after a reboot.