Update estimated traversal cost for BVH SAH
Update estimated traversal cost for BVH SAH

With the original .125 (vs intersection), we almost always made leaf nodes with ~1 primitive. Bumping up to 1 gives closer to 2 primitives on average in leaf notes, correspondingly smaller BVHs, and equivalent or slightly better performance (depending on the scene.)

nyue/pbrt-v3,mmp/pbrt-v3,bssrdf/pbrt-v3,nispaur/pbrt-v3,bssrdf/pbrt-v3,ambakshi/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,ambakshi/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,mwkm/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,bssrdf/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,ambakshi/pbrt-v3,shihchinw/pbrt-v3,mwkm/pbrt-v3