Use texture atlas for static icon item

Summary:
Icon's are generally small, they may as well go in the existing shared
texture atlas, save some memory and it means more items can be batched
whilst rendering.

The textures used during animation are unchanged for now as it also
requires changing FadingNode's shader.

Test Plan:
Ran plasma.
We use atlas's in frame/SVGItem already so it should be fine.

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: mart, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4342