Move XRenderUtils init/cleanup into X11 standalone platform

Summary:
Only needed for kwin_x11 variant (required for the non-composited
Outline). As that's nowadays in the x11 platform, we can move the
complete XRenderUtils support into the platform. Thus KWin core does
no longer require to link it.

Test Plan: Compiles

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7760