[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).
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