Matthew Chiu
4788808
mc77@stanford.edu

Hmw 1
=====
Implemented a 2D grid accelerator and added triangle meshes to
accelerator. IntersectClosest will directly intersect a ray with a 2D grid
that has triangle meshes inside. Made all of my changes to heightfield.cc
and heightfield.h. Ran it on hf*.rib. Seems to performance problems on
hf200x200.rib due to the amount of memory consumed by triangle mesh array.

