Use stale Sprites + JSON

[kkaefer]

With #2617 in place, we now get stale data from the cache. Currently, stale sprites and the associated metadata JSON isn't used, but we should change this. Things to consider:

• The easy case is when the refreshed request returns a 304. In this case, we don't have to do anything.
• The hard case is when we get a different response. Sprites that change may have different metrics. That means we'll have to reparse any layers that used icons
• The same is true for the Sprite Atlas: We need to update the Sprite stored in the atlas, and probably assign a different position. Note that this only applies when the actual dimensions of a sprite changed, or when there were sprites removed or added in the new data.

[kkaefer]

Implemented as part of #2721.