Add check for backwards-facing rays in RealisticCamera
Add check for backwards-facing rays in RealisticCamera

Fixes issue #92. Many thanks to @takiyu for chasing down the problem and
proposing a solution, and @syoyo for an alternative solution, which I've
incorporated here.

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