Compiles on linux.

Merge

Merge

Changed critical section for linux (sampler).

Added sampling (halton, stratefied, random), filter (mitchell, sinc, gaussian), integrator (path, direct lighting), and a rng from pbrt.

Changes+REFRACTIVE STACK IS BROKEN

Timer changes

Triangle intersection checks using ISPC

Adjusted happy scene. Shadow ray packet. Triangle collision detection in ISPC (UNTESTED). Fixed small BBox missing bug.

ISPC based hit testing

Fixing compile error on linux and crash due to mem pool

Packet tracing on primary and shadow rays

Memory Pool

Unnested loop and accurate timer

Passing BBox down the tree and reset the timers