Support Atlas textures in FadingNode

Summary:
Previously IconItem would create icons in new textures for animating. By
keeping them in the atlas not only do we avoid that, but we should
implicitly share the QSGTexture used for non-animating item.

Test Plan:
Moused over icon, it changed correctly
Set core profile, moused over an icon it changed correctly

Reviewers: Plasma

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11091