A median cut builder for Embree

Just for the sake of comparison, I have implemented a median cut builder. Not surprisingly, it has the fastest build times and the slowest rendering performance, although the gap with object splitting is narrower than I expected. 

To use this builder, specify -accel [bvh2, bvh4, bvh4mb].mediancut on the command line.

A note on the implementation: once you realize that a median cut is a special case of object spllitting with only two bins at all levels, coding the algorithm is really straightforward within Embree framework. Of course, the resulting implementation could have been made even simpler for improved build times but I was more interested into rendering performance comparisons.