[effects/showpaint] Modernize code

Summary:
Summary of changes:
* Use QVector to store colors
* Add Q_OBJECT macro
* Port away from QRegion::rects
* Fix coding style
* Use default member initialization
* Use nullptr
* Reserve rects in paintXrender

The behavior of this effect haven't been changed, it still acts as before.

Test Plan: Compiles, and the Show Paint effect is still a psychedelic thing.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15424