gsk: Turn GskRenderNode into a pure GTypeInstance

gsk: Turn GskRenderNode into a pure GTypeInstance

Using GObject as the base type for a transient tree may prove to be too
intensive, especially when creating a lot of node instances. Since we
don't need properties or signals, and we don't need complex destruction
semantics, we can use GTypeInstance directly as the base type for
GskRenderNode.