Fix bug in InfiniteAreaLight::Pdf_Le()
Fix bug in InfiniteAreaLight::Pdf_Le()

Negative angles returned from atan2() weren’t being remapped to [0,2pi].

Issue #62.

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