Use a better split strategy

[gkjohnson]

Right now, the BVH nodes are being split at the average point of all face vertices along the longest edge of the overall bounding box.

This allows for less-than optimal bounding boxes where there's a large gap between triangles instead of just containing the clusters of polygons.

[gkjohnson]

SAH seems to be the most well-regarded splitting approach.

http://thomasdiewald.com/blog/?p=1488
https://stackoverflow.com/questions/4632951/kdtree-splitting
https://www.keithlantz.net/2013/04/kd-tree-construction-using-the-surface-area-heuristic-stack-based-traversal-and-the-hyperplane-separation-theorem/

[gkjohnson]

Added an SAH, center, and average approach for the MeshBVH