Summary:
The Compositor now tries to create a Scene not just once but every
type supported by the Platform till it finds one which works. The user's
configuration is only used as a preferred hint and tried first if the
platform supports it.

This brings as an advantage that on platforms such as framebuffer the
user does not need to specify which compositor to use: KWin uses QPainter
automatically.

Also we don't need to do the "translation" from XRender to QPainter any
more. XRender is not supported by any platform using QPainter, so the
user configuration is ignored anyway.

BUG: 350159
FIXED-IN: 5.12.0

Test Plan: Run on framebuffer, verified debug output.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8363