AccessKit Disable GIFs: Blur animated images while processing

[marcustyphoon]

Description

Fun bit of CSS experimentation. This applies a temporary blur effect to images with GIF backgrounds that will be animated only for a moment while we're processing them. Like #1128 and #960, care is taken not to apply the effect when the extension is first loaded or the feature is switched on to avoid an unblurred -> blurred -> unblurred chain. Not sure this is really any better than not doing it, but it's cute.

Have you heard of the contain CSS property? I sure hadn't.

Branch based off of #1729.

Testing steps

[marcustyphoon]

This was supposed to be a just for fun thing, but, like... I really like it actually? :D But I think CSS blurs can perform really badly on older hardware, so my guess is it's too risky.

edit: Although Tumblr does use them in the Tumblr Premium modal and in the settings page when you click to edit your password (and used to(?) on "blurred" mature posts)... so maybe it's fine?

edit: This branch has been updated to be based on newer code, which (on sufficiently new browsers) pauses background-gif images more quickly by streaming the image response. At least on my internet connection, this makes this almost entirely pointless, as I hardly get a chance to see the blurred elements before they're paused.