The main idea behind the render backend is to decouple low level bits
from scenes. The end goal is to make the render backend provide render
targets where the scene can render.
Design-wise, such a split is more flexible than the current state, for
example we could start experimenting with using qtquick (assuming that
the legacy scene is properly encapsulated) or creating multiple scenes,
for example for each output layer, etc.
So far, the RenderBackend class only contains one getter, more stuff will
be moved from the Scene as it makes sense.
The GlStrictBinding flag indicates whether it's okay not to re-bind the X11
pixmap to the OpenGL surface texture if the corresponding window is damaged.
It doesn't really affect the SceneOpenGL, only low level backend stuff.
Currently, the scene owns the renderer, which puts more
responsibilities on the scene other than painting windows and it also
puts some limitations on what we can do, for example, there can be only
one scene, etc.
This change decouples the scene and the renderer so the scene is more
swappable.
Scenes are no longer implemented as plugins because opengl backend
and scene creation needs to be wrapped in opengl safety points. We
could still create the render backend and then go through the list
of scene plugins, but accessing concrete scene implementation is
much much simpler. Besides that, having scenes implemented as plugins
is not worthwhile because there are only two scenes and each contributes
very small amount of binary size. On the other hand, we still need to
take into account how many times kwin accesses the hard drive to load
plugins in order to function as expected.
This allows using base opengl backends in libkwin, which can be useful
later on for the purpose of moving the ownership of render backends from
the Scene class to the Compositor class.
It has been disabled with Mesa for almost half a decade due to false
positives and even if it weren't disabled, it contributes to the startup
time.
The commit message that added the self test doesn't explain why it was
added, but if it was added to detect unstable drivers, it's not worth it.
