don't leave desktop unobscured when it's covered
don't leave desktop unobscured when it's covered

...after the opening animation has finished.  Test case:
1. map some window
2. go back to switcher
3. activate the window
6. the window is stacked, but it isn't considered covering the desktop
   because it's animating
7. later on visibilities are not checked once again because the stacking
   doesn't change anymore

  * src/mcompositemanager.cpp (onAnimationsFinished):
    Recheck visibilities then.

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