Workweek displaylist

Conversation about displaylist construction in parallel

• Change assign-heights to also keep track of the number of display list items that each child will create and where they need to be created.
• Then, we can preallocate the displaylists before we do displaylist creation and tell the children where each of their display items should be inserted.
• We're currently spending a lot of time building the displaylist because we do not take the viewport into account. Gecko does well with that optimization.