clutter: Make clutter_actor_create_texture_paint_node public
Since StDrawingArea in gnome-shell is the only user of ClutterCanvas, it is possible to move ClutterCanvas completely out of Mutter to gnome-shell. This allows to remove another Cairo dependency from Mutter.
This patch makes clutter_actor_create_texture_paint_node() function public to be used by StDrawingArea in gnome-shell that replaces ClutterCanvas.