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.

r? @pcwalton
Fixed the inline_element_border_a reftest with incremental layout turned on.
auto merge of #3706 : cgaebel/servo/fix-image-dynamic-remove, r=pcwalton

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