Implement InfiniteAreaLight

[wahn]

$ rg -tcpp 'public Light' ~/git/github/pbrt-v3/src
...
/home/jan/git/github/pbrt-v3/src/lights/infinite.h
52:class InfiniteAreaLight : public Light {
...

[wahn]

After commit b0ccd14 we start to see the contribution of an InfiniteAreaLight (test scene comes from issue #24):

What's missing is the color for rays which do not hit any geometry.

[wahn]

Using the proper light flag (see commit 6738bfa) is all we needed: