GLPlatform can recommend the compositor to use

During detecting the OpenGL capabilities also a recommended compositor
can be set. This recommendation is honoured by the OpenGL based
compositors. That is the SceneOpenGL2 requires a recommendation for at
least OpenGL2 and the SceneOpenGL1 requires at least a recommendation for
OpenGL1. If the driver recommends XRender compositing the SceneOpenGL
performs the existing fallback to XRender.

With this recommendation the hacks in the Scene are removed, e.g. it is
no longer checked whether the driver is software emulation as that is
provided through the recommendation.

To overrule the recommendation the environment variable KWIN_COMPOSE is
extended by the values O1 and O2 to enforce either OpenGL 1 or OpenGL 2.
This overwrites all other checks. As a side-effect this allows now to run
KWin on the llvmpipe:
LIBGL_ALWAYS_SOFTWARE=1 KWIN_COMPOSE=O2 kwin --replace &

But not that I would recommend to use it :-)

REVIEW: 106741