After the splitting-into-lines pass, remove any trailing whitespace
from all of a block's line boxes.
This improves the appearance of text-align: justify/right :^)
In order for this to work nicely, I made the line box classes use float
instead of int for its geometry information.
Justification works by distributing all of the whitespace on the line
(including the trailing whitespace before the line break) evenly across
the spaces in-between words.
We should probably use floating point (or maybe fixed point?) for all
the layout metrics stuff. But one thing at a time. :^)
Inline layout is now done by LayoutBlock. Blocks with inline children
will split them into line boxes during layout.
A LayoutBlock can have zero or more LineBox objects. Each LineBox
represents one visual line.
A LineBox can have any number of LineBoxFragment children. A fragment
is an offset+length into a specific LayoutNode.
To paint a LayoutBlock with inline children, we walk its line boxes,
and walk their fragments, painting each fragment at a time by calling
LineBoxFragment::render(), which in turn calls the LayoutNode via
LayoutText::render_fragment(). Hit testing works similarly.
This is very incomplete and has many bugs, but should make it easier
for us to move forward with this code.
