Don't reload background contrast effect on screen resize
Summary:
Similar to the Blur patch. Arguably rven less reason as this doesn't
have a texture the size of the workspace.
We don't need to delete and recreate the entire effect every time the
screen changes; This deletes the wayland global which causes quite a bit
of extra work for the clients.
Test Plan:
Changed resolution under X, and scale under wayland
Panel looked the same and nothing exploded
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: graesslin, plasma-devel, kwin, KWin
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D7937