Fix emoji sprite offset in Firefox and Chrome

[kristinashu]

On desktop, the super creepy face disappears at the very bottom, you should still be able to see it.

On mobile, I see the happy face at the very very bottom of scrolling, should see the super creepy face.

[kristinashu]

Doesn't look fixed on staging or prod.

[Pomax]

It would appear that this is a mismatch between Chrome and Firefox, where:

• chrome: keeps the face, does not seem to compute the sprite offset correctly (it seems to stop at -1610px, which is about 1000px off the mark)
• firefox: computes the sprite offset correctly, but is off by one for the most-creepy frame it looks like (ending at -2800 rather than the -2660 for the last frame)

[kristinashu]

😬