actor: Add private getter for the active framebuffer

Instead of asking every internal user to get the stage and get the
active framebuffer from it, we can wrap it up ourselves, and do some
sanity checks as well.
Use the non-deprecated Cogl clipping API

Cogl 1.18 deprecated the global clipping API in favour of the
per-framebuffer one, but since we're using the 2.0 API internally we
don't have access to the deprecated symbols any more.

This is pretty much a mechanical port for all the places where we're
still using the old 1.x API.
text: Add a hacky fallback for the framebuffer

The text-cache conformance test breaks because ClutterText gets a paint
without an active framebuffer associated to the ClutterStage. Keep a
fallback while we investigate the issue.
paint-node: Get the framebuffer from the root node

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.
conform: Run texture tests only on -m=slow

I don't want to remove them altogether, but they need to be ported to a
more reliable system, otherwise they end up failing at random depending
on the whims of the compositor and the windowing system.