Instead of calling reconfigure() on screen size changes, which deletes
and recreates the effect, we can just update the one texture that's mapped to
the screen size.
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From the wayland commit.
>What surprises me is that BlurManager recreates for Output changes.
>That sounds like a bug in KWin (or an area which could be improved).
I had thought the same and wrote this (though wanted to have kwayland first)