Fix jank when using full height images, and other misc media preview imrpovements

[hjiangsu]

Pull Request Description

This PR fixes an issue with layout jank that occurs when using full height images and a few other minor issues.

• Fix layout jank due to thumbnailUrl not being present in some occasions
• Added height values to ViewMode enums to remove our scattered usage of constants
• Added a timeout when attempting to fetch image dimensions - this should help with some loading times if the response times of the image URL is slow. This does mean that some images may look squished or stretched if a timeout occurs but it might be worth it for faster loading. Ideally, the Lemmy API should provide the height/width dimensions so that we don't have to manually fetch these dimensions
• Added a background color to the image container when the image has not loaded yet

If any image dimensions look off after this change, let me know!

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[micahmo]

Hmm, I tried testing these changes and right off the bat the feed is filled with stretched images. Is it worth the tradeoff of faster loading?

[hjiangsu]

Nice catch! I didn't test that out. I'll check it out and see - the stretching should only happen on card view

[hjiangsu]

Hmm, it looks correct on my device. I'm not sure what's causing the stretching. I'm out at the moment so I can't check on the emulator!

[hjiangsu]

Just tested it on Android as well and ti looks okay to me. Could you double check if you can reproduce the stretching?

[micahmo]

Alright I'm not sure what was going on, but I can't repro any more either!