stb_image: Add support for gainmaps (libultrahdr) support to image loader

[cmahnke]

Is your feature request related to a problem? Please describe.
This is a feature request, see Tom94/tev#226, which uses sbt_image internally.

Describe the solution you'd like
Loading HDR images, in the form of JPEG files with gainmap should be supported. The format is implemented by libultrahdr, a technical description is linked in the further ressources section.

Describe alternatives you've considered
None yet, since this is upstream library.

Additional context
Further Resources:

• Description of the format
• Using libultrahdr with Python
• Greg Benz on HDR

CC @gregbenz

[nothings]

We're not adding any new image formats to stb_image.

[cmahnke]

It's not a completely new format, it's just a SDR JPEG with an additional data stream, containing the gainmap.