gsk: Rework GskRenderer and GskRenderNode semantics

gsk: Rework GskRenderer and GskRenderNode semantics

This commit changes the way GskRenderer and GskRenderNode interact and
are meant to be used.

GskRenderNode should represent a transient tree of rendering nodes,
which are submitted to the GskRenderer at render time; this allows the
renderer to take ownership of the render tree. Once the toolkit and
application code have finished assembling it, the render tree ownership
is transferred to the renderer.