Initial implementation of GSK rendering pipeline

Initial implementation of GSK rendering pipeline

GSK is conceptually split into two scene graphs:

 * a simple rendering tree of operations
 * a complex set of logical layers

The latter is built on the former, and adds convenience and high level
API for application developers.

The lower layer, though, is what gets transformed into the rendering
pipeline, as it's simple and thus can be transformed into appropriate
rendering commands with minimal state changes.

The lower layer is also suitable for reuse from more complex higher
layers, like the CSS machinery in GTK, without necessarily port those
layers to the GSK high level API.

This lower layer is based on GskRenderNode instances, which represent
the tree of rendering operations; and a GskRenderer instance, which
takes the render nodes and submits them (after potentially reordering
and transforming them to a more appropriate representation) to the
underlying graphic system.