Instead of best-guessing, at BGR (which in retrospect was a bad guess),
offer whatever resembles most the internal representation. This way the
frame gets to be least treated as it goes into the client.
Move some generic properties to the OutputLayer class and add a cursor
layer getter in the RenderBackend class. That allows us to get rid of
some code duplication. And we could use this work to move more cursor layer
code from backends to Compositor.
Things such as Output, InputDevice and so on are made to be
multi-purpose. In order to make this separation more clear, this change
moves that code in the core directory. Some things still link to the
abstraction level above (kwin), they can be tackled in future refactors.
Ideally code in core/ should depend either on other code in core/ or
system libs.
