Apart from removing a lot of code, this commit also introduces (thanks
to wlr_scene) fine-grained damage tracking.
The lack of our own rendering code will only be an issue if the need for
custom effects (such as blur) arises; however, that's very unlikely and
it would probably require a lot of changes anyway.
At the moment it isn't used at all and a lot of data is duplicated. This
is a temporary state introduced for nicer diff separation into several
commits.
