Since Cogl has started restricting what cogl 1.x api is exposed when
COGL_ENABLE_EXPERIMENTAL_2_0_API is defined and since we build all
Clutter internals with COGL_ENABLE_EXPERIMENTAL_2_0_API defined this
patch makes a first pass at reducing our internal use of the Cogl 1.x
api.

The most notable api that's no longer exposed to us internally is
the cogl_material_ api so this switches all Clutter internals to use the
cogl_pipeline_ api instead. This patch also makes quite a bit of
progress removing internal uses of CoglHandle although there is still
more to go.