Add get_type() functions for GskRenderNode subclasses

Add get_type() functions for GskRenderNode subclasses

The introspection scanner tries to match a type name with a get_type()
function, in order to correctly identify a type as a class.

If the function is not available, we have two choices:

 - add some special case inside the introspection scanner, similar to
   how GParamSpec subclasses are handled in GObject
 - add a simple get_type() function

The latter is the simplest approach, and we don't need to change that
much, since we still register all render nodes at once.