Fix a rare NaN in TrowbridgeReitz sampling
Fix a rare NaN in TrowbridgeReitz sampling

If the outgoing direction was exactly on the tangent plane (i.e., wo.z == 0), TrowbridgeReitzSample11() would end up generating an infinite slope value.

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