This is a part of refactoring towards making the paintable tree
independent of the layout tree. Now, instead of transferring text
fragments from the layout tree to the paintable tree during the layout
commit phase, we allocate separate PaintableFragments that contain only
the information necessary for painting. Doing this also allows us to
get rid LineBoxes, as they are used only during layout.
When calculating the width of text using a bitmap font, a glyph spacing
is added at the end of each fragment, including the last one. This meant
that everything was 1 pixel too long. This bug did not affect vector
fonts.