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.
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 fixed first)