store contrast matrix per-window

Summary:
as different windows can have different contrast region parameters,
store color matrices per window
BUG:339237

Test Plan: logout window doesn't break panel anymore

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: broulik, plasma-devel, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D5048