Do not detect Ultra HDR images as MPO

[radarhere]

Resolves #8052

The Ultra HDR format stores its gain map as an additional MPO image. However, as the issue has found, because we do not support Ultra HDR images, the additional image appears incorrectly.

Instead, this PR treats such files as standard JPEGs.

https://developer.android.com/media/platform/hdr-image-format#signal_of_the_format details how this format can be detected.

A JPEG file conforming to this format may be identified by the presence of hdrgm:Version="1.0" in the primary image's XMP packet

[hugovk]

Tests/images/ultrahdr.jpg is about half a megabyte, not our biggest test image (which is 9.9 MB!), but still pretty big.

Can we find a smaller image to include in the test suite? cc @bigcat88

[bigcat88]

I am not the author of that image.

Here is the link to the message of author #8052 (comment)

where he mentions that he does not have the smaller image (

[radarhere]

@hugovk would you prefer I create a smaller image artificially?

[hugovk]

If it's not too difficult and it doesn't feel too hacky? Otherwise the one already here is probably fine. Just keeping an eye on our 46.6 MB sdist.

[radarhere]

I've realised that since we're not loading the image, I can just truncate the file leaving only the data that's read in the initial open. The test image is now down to 48kb.