This fixes the visual artifacts seen at the top of wikipedia pages by:
- Setting clipping rect to avoid images going outside their bounds.
- Handling case of background-position being >= 100% such that wrapping is required.
However, the gradient is not currently visible on wikipedia. This relies on bug #1997 being fixed.
When calculating the preferred width for a block, accumulate
the left and right float widths of children separately, which
is then max'ed with the normal flow widths later on.
Ref bug #2554 - improves the layout of the top bar.
