vsFrustum can now classify box visibility more thoroughly.

Previously, only determined whether boxes were outside.

Now can return whether boxes are entirely-inside, partially-inside, or
entirely-outside.

Also adds a sphere classification function to the vsFrustum, which is
tested by the box classification function before testing the box, just
since testing against spheres is a lot simpler and faster, and can often
give us the box classification without going to the full expense.  This
gave me a huge performance boost in MMORPG Tycoon 2, which makes heavy
usage of quad trees for rendering.

Also adds a utility function for accessing this information on the
vsCamera3D.

TODO is to update the vsOctree sample implementation to use these new
visibility values to optimise its drawing code.