AccessKit Disable GIFs: Fix unstretched photoset hover area

[marcustyphoon]

Description

Sufficiently-narrow images are not stretched horizontally to the post container width on desktop Tumblr. This fixes a minor issue with our current code where the hover-to-animate area of a disabled unstretched GIF included the blank space to its right and any adjacent non-animated images, because we were processing them as if they're part of a photoset.

Note that at time of writing I left a bug in this: resizing the viewport across the desktop <-> tablet boundary in either direction results in slightly unintended photoset hover behavior, though it's no worse than before. If anyone can think of an elegant fix, be my guest, but otherwise, eh, I doubt anyone will notice.

Testing steps

• Open https://www.tumblr.com/transienttest/775813961932324864/one-gif-two-gifs-red-gif-whatever-i-cant-be with the desktop viewport width. Confirm that hover areas are correct: unstretched images are unpaused when hovered; "photosets" are unpaused when any part of them are hovered.
• Resize the viewport to the tablet or mobile viewport width. Observe that hovering is... good enough.
• Open https://www.tumblr.com/transienttest/775813961932324864/one-gif-two-gifs-red-gif-whatever-i-cant-be with the tablet or mobile viewport width. Confirm that hover areas are correct: "photosets" are unpaused when any part of them are hovered.
• Resize the viewport to the desktop viewport width. Observe that hovering is the way it was without this PR.