Gainmap metadata

[awxkee]

You better check this with care, I'm not the very best in JPEGs parsing

[awxkee]

Perhaps we could change pub gain_map_info: Vec<GainMapInfo>, to pub gain_map_info: Option<GainMapInfo>, if there is no plans to parse the whole image at one time ?
It doesn't look like is expected on libultrahdr to have multiple gain map metadata stored in the one image.

[awxkee]

Here is MPF parser, looks like everything works.

Except that secondary image offset point before the first image ends, however images sizes is correct, so probably offset computed from somewhere not at start from the file.