The PaintNode hierarchy should have the ability to retrieve the
current active framebuffer by itself, instead of asking Cogl using the
global state API.

In order to do this, we ask the root node of a PaintNode graph for the
active framebuffer. In the current, 1.x-compatibility mode we have two
potential root node types: ClutterRootNode, used by ClutterStage; and
ClutterDummyNode, used a local root for each actor. The former takes a
framebuffer as part of its construction; the latter takes the actor that
acts as the local top-level during the actor's paint sequence, which
means we can get the active framebuffer from the stage associated to the
actor.

By keeping track of the active framebuffer on the node themselves we can
drop the usage of cogl_get_draw_framebuffer() in their implementation.