- Use the border box of the floated element when testing if something
needs to flow around it.
- Take the floated element's containing block size into account (instead
of the BFC root) when calculating available space on a line where a
right-side float intrudes.
When we decide that a box has definite width or height based on its
containing block's corresponding size, we'll want to resolve the
current box's size as well. Otherwise anyone querying the size on
this box will get the bogus message of "yes, this definite, and its
value is zero."
This state is less static than we originally assumed, and there are
special formatting context-specific rules that say certain sizes are
definite in special circumstances.
To be able to support this, we move the has-definite-size flags from
the layout node to the UsedValues struct instead.
