[fluxus] depth sorting commit

[dave]

I've added (hint-depth-sort) which tags objects to delay rendering until
after the rest of the scene graph, sorts these objects according to
depth from the camera and renders them in order.

This is needed to automatically render transparent objects correctly -
it's a slower rendering path though (does a matrix multiply and some
memory allocation per object/per frame) hence it should only be set on
transparent objects (alpha textured or with opacity set).

cheers,

dave