Fix image_dynamic_remove reftest with incremental layout turned out

This also adds some extra debugging infrastructure which I found useful tracking
this bug down. A regression in the br reftests is also uncovered by this patch,
which I'll work on fixing next.

EDIT: nevermind. no regression, I just tested that before a rebase.

r? @pcwalton
It need to be mutated during line-breaking, and since TextRuns are shared behind an Arc, it'd make the entire TextRun need a clone.