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.