Add support for Ultra HDR

[trlef19]

It would be nice to have

[grishka]

(disclaimer: the newest Android device I own is a Pixel 4a and it doesn't support any of this so I can't test how Android handles it)

Ultra HDR needs extra metadata to display as, well, HDR. Right now, Mastodon strips that metadata, so after being processed by Mastodon, the image loses its Ultra-HDR-ness, as tested in Vivaldi on a MacBook Pro with one of these images (the highlights in the original files appear brighter than the #FFFFFF UI white when viewed in a Chromium browser, and after processing it becomes just a regular image).

So, I suggest that you create an issue in the main Mastodon repo about preserving that metadata during image processing. After that's done, I believe it should only be a matter of me setting some parameters to get the image viewer window to use the right display mode on devices that support it.

[trlef19]

Did it. Thanks