Make progress bar less erratic

Solves the bug, and makes the code slightly cleaner, but it
still does not inspire confidence (e.g. use of viewer flags
by a bunch of callbacks that can run for a background image).
Also, the tests seem underspecified.

I'll follow up with some more refactoring.

Change-Id: I2557abcec173691ffce21185bf1a939f1644ba8c
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/489
Merge "Make progress bar less erratic"