More Clutter API

This is a bunch of API additions derived from making MetaShapedTexture a ClutterContent implementation. I'm splitting this out to ease review burden and allow us to move towards ClutterContent in small, well defined and sane steps.

Three additions were made:

• clutter_content_invalidate_size(): necessary for when a ClutterActor has CLUTTER_REQUEST_CONTENT_SIZE request mode, and the ClutterContent changed its preferred size. Currently, there is no way to notify these actors about size changes.
• clutter_paint_node_add_multitexture_rectangle(): this is necessary for wrapping cogl_framebuffer_draw_multitextured_rectangle() calls. They are not equivalent to multiple calls to cogl_framebuffer_draw_textured_rectangle() -- the former uses different layers of the pipeline, the latter uses only the first layer.
• ClutterRootNode: it is a paint node that is the top-most node on a paint node tree.