x11: listen to xrandr events

If the Xrandr configuration changes, re-read it. So if you change
display modes or screen configuration, it will update the framedrop
refresh rate accordingly.

This passes the rootwin to XRRSelectInput(), which may or may not be
allowed. But it works, and the documentation (which is worse than used
toilet paper, great job Xorg) doesn't forbid it, or in fact say anything
about what the window parameter is even used for.