HDR gain maps

[neckaros]

Now that this library can manage additional metadata like ICC profile and orientation.
What is the the strategy on gain maps / transfer functions ?
They are more and more used with the advance of HDR capable display and now most phones will take HDR photo and save them using gain maps for retro-compatibily on SDR screens

For some context here is the discussion for the implementation that has been done in ImageMagick: ImageMagick/ImageMagick#6377

Features that could be implemented:

• Like for ICC profile: get and set hdr gain maps (each encoder / decoder will need to implement it)
• allow conversion of sdr image to HRD using a gain map as input