Introduce render layers
This is the first tiny step towards the layer-based compositing in kwin. The RenderLayer represents a layer with some contents. The actual contents is represented by the RenderLayerDelegate class. Currently, the RenderLayer is just a simple class responsible for geometry, and repaints, but it will grow in the future. For example, render layers need to form a tree. The next (missing) biggest component in the layer-based compositing are output layers. When output layers are added, each render layer would have an output layer assigned to it or have its output layer inherited from the parent. The render layer tree wouldn't be affected by changes to the output layer tree so transition between software and hardware cursors can be seamless. The next big milestone will be to try to port some of existing kwin functionality to the RenderLayer, e.g. software cursor or screen edges.