Since paintables have a default content size of 0x0, we were neglecting
to notify the corresponding layout node about size changes, if the used
content size came out to 0x0.
This fixes an issue where resizing an iframe to 0x0 didn't take effect.
No need to call the expensive establishes_stacking_context() here, as
we've already built the stacking context tree and can simply test for
the presence of existing stacking contexts.
The absolute rect of a paintable is somewhat expensive to compute. This
is because all coordinates are relative to the nearest containing block,
so we have to traverse the containing block chain and apply each offset
to get the final rect.
Paintables will never move between containing blocks, nor will their
absolute rect change. If anything changes, we'll simpl make a new
paintable and replace the old one.
Take advantage of this by caching the containing block and absolute rect
after first access.
