The benefits of this:
- One code path for determining spinner/placeholder and it's position
for loading images/stickers. This includes spinner used in e2e
decryption of images.
- Very small definition for MStickerBody, only overriding the minimal
differences is has from MImageBody.
The disadvantages:
- Slightly more complicated MImageBody, but hopefully not less
readable.
As the slightly nicer alternative to fixupHeight being applied once
we actually have a timelineWidth.
The niceness comes from not needing timelineWidth, which means we can
implement at render time with CSS. (Despite still calculating aspect
ratios when we render.)
