Don't try to resize desktop or fullscreen windows

Summary: those windows should always take all the space no matter what

Test Plan: text areas on desktop or fullscreen windows don't cause resizes anymore

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D19922