Summary:
Creating the OutlineVisual is moved into the Platform API. The default
implementation creates the composited OutlineVisual. The X11 standalone
platform overrides it and creates the non composited outline in case no
compositing is used.

Test Plan:
Run kwin_x11 with KWIN_COMPOSE=N and KWIN_COMPOSE=X,
non composited outline and composited outline loaded

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7450