kwin: Optimizing Blur Part III

This patch introduces some kind of damage propagation. In the old version of the blur effect
we had to repaint the whole window if sth behind the blurred area was damaged. The new texture
cache, which was introduced by the last patch, gives us the opportunity to only update parts of
blurred background texture. This means that the damaged area can only propagate with the speed
of the blurring radius per window layer.

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