It's a premultiplied image. This implies that we were misusing encodePNG
in most cases, as we were passing premultiplied pixels which were then
interpreted as unmultiplied. I changed encodePNG to accept premultipled
pixels, and unpremultiply in the implementations.
when passing a std::string instead of an Image, the parameter gets implicitly and automatically converted (== parsed) to a temporary Image object. Since parsing an Image shouldn't be a frequent operation
