BDPT: only trace shadow ray to light if path has non-zero contribution
BDPT: only trace shadow ray to light if path has non-zero contribution

No change in images; roughly 5% speedup.

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