Bug 1003843 - Follow up to Bug 951676 - [Messages][Refresh] Add specific image in case of more than one recipient in the list threads. r=schung

yeah, I'm not sure the complexity is worth it, especially that it will be invoked at startup and may incur a sync reflow.

For this I'd rather take a magic number as we don't need to be precise. What do you think?

(thanks for showing me getBoxQuads though, I missed this new API :) )