Don't recreate kwayland blurmanager on screen size changes

Summary:
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 fixed first)

Test Plan:
Logged in and out (my screen resizes on startup due to scaling)
Tested on my X machine by turning a screen off. Nothing broke, plasma
panels looked the same.

Note: Eike tried this patch last week and claimed kwin still crashed.
We don't have a backtrace so it could have been something unrelated,
applied wrong or that he was deliberately lying to wind me up.

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: graesslin, plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7877