[effects] Add effect to hold the desktop window static after plasmashell quits

Summary:
When a plasma session quits, the order of plasmashell and clients quitting is undetermined.

This looks bad as our desktop background disappears quite early. Rather than changing the real order, we can have kwin reference the last pixmap of any desktop and hold on to it.

Lasting till the compositor quits means it stays on the X frontbuffer till X quits and the experience is seamless.

Test Plan: Logged out (with relevant other ksmserver patches)

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24864