[Effects/blend] Let fullscreen effects take priority

The Blendchanges effect works by showing a snapshot of the current
screen for a few ms. Any other active full screen effect will look
broken, so it should take priority.

This is pre-empting an issue with a new Plasma feature that syncs
colours to wallpapers. Once we animate activity switches / have per
desktop wallpapers it'll glitch out.


(cherry picked from commit e23c3a69)