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.
The freetype hinting only works on integer pixel sizes. For this
reason, the advance width metrics for a font of size 12.99 are
the same as the advance metrics for a font of size 12.0. This
results in small fonts appearing to overlap slightly, which is
particularly noticeable on parts of Wikipedia. Round the font
size up to a pixel boundary inside the freetype system.
Also fetch the system default fonts for the generic font families
rather than hard coding them.
These two changes make the font rendering on Linux very close
to the Firefox font rendering on Wikipedia.
