Previously the page background was always draw relative to the viewport
instead of following with the content. This should eventually become
an opt-in mode (via CSS "background-attachment") but for now let's have
the default behavior be that backgrounds scroll with content.
Also take this opportunity to move the background painting code from
the two web views to a shared location in InitialContainingBlockBox.
Image boxes want to know whether they are inside the visible viewport.
This is used to pause/resume animations, and to update the purgeable
memory volatility state.
Previously we would traverse the entire layout tree on every resize,
calling a helper on each ImageBox. Make those boxes register with the
frame they are interested in instead, saving us all that traversal.
This also makes it easier for other parts of the code to learn about
viewport changes in the future. :^)
