stage-window: make it possible to damage the back buffer

This allows us to report to the backend that the stage's back buffer has been trashed
while handling picking. If the backend is keeping track of the contents of back buffers
so it can minimize how much of the stage is redrawn then it needs to know when we do pick
renders so it can invalidate the back buffer.

Based on patch from Robert Bragg <robert@linux.intel.com>

https://bugzilla.gnome.org/show_bug.cgi?id=669122