Add support for Ultra HDR

[cmahnke]

I would like to suggest to add the addition of the Ultra HDR format as a new way to read and write HDR images.

From a technical point of view it's similar to the MPO format, that it's a JPEG image, containing an additional data stream. In case of Ultra HDR this data stream is a gainmap.

This way a reader implementation not aware of Ultra HDR only sees the SDR JPEG image.

There are currently several open enhancement requests to add the format to various well (and less well known) libraries and tools:

• LibVips #3799
• ImageMagick #6377
• Tev #226

There is also a library