There is only one file in this folder that is in scope of
GSoD - image_decoder.h. It was in great shape docwise though,
so not much review required.
The rest of the headers have received a doxy group definition.
Previously, the PNGDecoder would always convert images that
have a palette with a transparent color(s) to a full RGBA
surface automatically. There needed to be a way to diable
this and keep the image paletted for the Glk engine, since
some Infocom V6 game Blorb files reuse the palettes from
previous images, so I couldn't have the decoder using the
dummy palette that comes with the image