Step two: rename the object and its methods.
While we're at it, adhere more strictly to the coding style
practises; rename :clone-source to :source; add a setter method
for the :source property; take a reference on the source actor
to avoid it disappearing while we're still accessing it.
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.