Avoid copying drawables and dirtying display lists.

Avoid copying drawables and dirtying display lists.

For the alpha-test animation, use an OVERRIDE attribute on the state
set of the top level node instead of copying drawables and state sets
throughout the model.

As a temporary hack in the blend animation, don't use display lists in
the cloned drawables.

These changes are aimed at cutting down the number of display lists
that the pager needs to compile.