kwineffects: add API for modifying the blur and contrast regions of windows

This allows for effects that modify the window geometry to also adjust the blur/contrast region to match the modified window geometry.

With !2982 (merged) effects that did this by adjusting the WindowBlurBehindRole data directly broke, this adds the functionality back. Additionally this allows for much more dynamic adjustments, like possibly adjusting the blur region behind a wobbly window.