Handle ray differential perpendicular to tangent plane case in ComputeDifferentials().
Handle ray differential perpendicular to tangent plane case in ComputeDifferentials().

mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3