- cancelled previous commit to add RE_findTFAce, instead just added
a MemArena to render struct... free'd at end of render, can be used
to store other data as well
- switch rendering to using DerivedMesh API... this is slightly more
inefficient now because it is doing some unnecessary copying. Can
be fixed by defining a DerivedMesh function to convert the object
into a render object (on todo list)