Perform glFlush before deleting the EffectFrame's pixmaps

On NVIDIA it is possible that the actual rendering gets delayed to
after the deletion of the pixmap during the end of fullscreen effects.
This was causing freezes. By using glFlush before deleting the pixmaps
we can ensure that the pixmap is not needed anymore after the pixmaps
are deleted.

BUG: 261323
FIXED-IN: 4.6.3