[effects/cube] Update cubecap texture

Summary: Current cubecap texture is an old KDE logo. Let's use something newer.

Test Plan:
Before:

After:

Before:

After:

Reviewers: KWin, Plasma, VDG, davidedmundson

Reviewed By: KWin, Plasma, davidedmundson

Subscribers: davidedmundson, ngraham, broulik, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D15726