Render image diffs as abstract blocks diffs via DocumentEngine

Summary:
Depends on D20830. Ref T13425. Have the image engine elect into block rendering, then emit blocks.

This is rough (the blocks aren't actually diffed yet) but image diffs were already pretty rough so this is approximately a net improvement.

Test Plan: Viewed image diffs, saw nothing worse than before.

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20831