This fixes transparency for the slider widget handles in v6-7 games.
Someone may want to adjust the image decoder later, but it will do for
now. Not all icons have useful masks, but some do.
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.