meta-surface-actor-x11: Fix unredirect heuristic

Damage coordinates are relative to the drawable not to the screen. So we
have to check whether x and y are 0 and not window_rect.x/y otherwise the
herustic will never trigger for windows on monitors whos x and y are not 0.

https://bugzilla.gnome.org/show_bug.cgi?id=738271