[stage-x11] Fix switching fullscreen mode

[stage-x11] Fix switching fullscreen mode

Setting/unsetting fullscreen on a mapped or unmapped window now works
correctly.

If you unfullscreen a window that was initially full-screened, it will
unset the fullscreen hint and the WM will likely push the size down to
the largest valid size.

If the window was previously un-fullscreened, Clutter will restore the
previous size.

Fullscreening also now works if the WM switches the hint without the
application's knowledge (as happens when you resize a window to the size
of the screen, for example, with stock metacity).