Summary:
Slided popups are clipped twice:
* first, when filtering window quads in prePaintWindow;
* then, when doing scissor test (the opengl scene plugin does scissor
test for transformed windows).

Because of that, we don't need to filter window quads. Filtering window
quads only adds overhead.

This change simplifies code and "fixes" incorrect clipping when a slided
popup has shadows.

Test Plan:
* Put Plasma panel on the left screen edge;
* Open and close the Application Launcher.

(repeat for top/right/bottom screen edge)

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14450