AccessKit Disable GIFs: Remove/update "download immediately/download on hover" feature

[marcustyphoon]

It appears that Tumblr has changed the GIF-displaying code to no longer display a stationary poster element, then hide it once the animated GIF it is for has been loaded. Instead, they're swapped (or something; I can't quite tell) using javascript. Presumably whoever made this change ensured that there will be no flickering effect on image load when this happens...? I hope?

This breaks the XKit code, added in #1853, which a) makes disabling most GIFs quite a bit faster and more efficient and b) adds a feature to not download them until they're hovered over. Disable GIFs still works, though (using our old canvas-based code path that was used before this change for webp images).

I don't currently have any ideas how to keep the "don't download" feature if Tumblr keeps this change, but I'm leaving it in the issue title because, like, idk, never say never.

I think it probably makes sense to not assign or helpwanted this issue and let the stale bot serve as an automatic remindme. We should at least see if the change is an a/b or a mistake or something (it hasn't been announced in changes yet).

[marcustyphoon]

Slightly weird update: as noted in #2247, Tumblr now has an option to disable media autoplay that pauses GIFs until you click them... and what's interesting is that their version (currently) waits to download the animated version until you click it. So we technically have the capability of implementing this toggle again... if and only if the user has their Tumblr set to disable media autoplay. (Specifically, we'd simulate clicking the image either when we process it, or on first hover).

However, unless we can hack the tumblr frontend a bit, the preference toggle would do nothing (it would effectively always be "immediately") if the user had media autoplay enabled, as is the default (usually), and we don't have the preference pane area to explain this. So—to my disappointment—I don't currently have a practical idea about how to keep the feature.

Side note: I just submitted feedback to Tumblr about this, but their feature is missing a loading indicator when you've clicked to unpause a GIF and it hasn't finished downloading, and it can take quite a long time and just looks like the GIF is not animated in the meantime. If Tumblr doesn't fix this, and we do build #2247's compatibility feature, I think we would actually potentially want to simulate a knightriderloader (ugh). This has been fixed!