Summary:
Lanczos filter shouldn't be in KWin core because it's quite specific about
rendering backend.
Test Plan: Windows in the desktop grid look normal.
Reviewers: #kwin, davidedmundson
Reviewed By: #kwin, davidedmundson
Subscribers: mart, kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D18774
