Mark windows changed on Expose

On i3wm, windows aren't unmapped when switching away from a window's
workspace, but it does cause OBS to lose the capture. Because
switching back will not trigger a MapNotify, the capture fails to
restart unless you resize or move the window (ConfigureNotify). An
Expose event is fired by the wm, however, so catching this correctly
restarts the capture.
Merge pull request #314 from raincomplex/master

Mark windows changed on Expose