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