This simplifies the mucking about with the model-view matrix that was previously
done which improves its efficiency when scaling is necessary.
Notably: There should now be no performance advantage to using
ClutterCloneTexture as a special case clone actor since this method is just as
efficient.
The unit test was renamed to test-actor-clone.
Many use cases for clonning an actor don't require running a shader on the
resulting clone image and so requiring FBOs in these cases is overkill and
in-efficient as it requires kicking and synchronizing a render for each clone.
This approach basically just uses the paint function of another actor to
implement the painting for the clone actor with some fiddling of the model-
view matrix to scale according to the different allocation box sizes of
each of the actors.
A simple unit test called test-actors2 was added for testing.
