Notes on C++ implementation.

Filter::invRadius is only used in mitchell.cpp, but it is recomputed in FilmTile::FilmTile, why not pass the filter and use it directly?
Precomputing halfPixel versus using Vector2f(0.5f, 0.5f) is inconsistent.
Why is Film::pixels a std::unique_ptr<Pixel[]> but FilmTile::pixels is a std::vector<FilmTilePixel>, why not the same for both?
Why isn't LOOKUP_ONE and LOOKUP_PTR used for FindOneFloat and FindFloat.

Pull requests:

mmp/pbrt-v3#295
- Why does ParamListItem have isString, it's not used.
- Why do parseParams and AddParam take a SpectrumType, it doesn't appear to be used.