[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