We use Harfbuzz to lay out glyphs at their right positions, but we
should not do so for substrings as we were doing for selection/range
rects: their positioning is dependent on their full context to properly
determine kerning and ligatures, for example.
Instead, use the stored glyph positions & widths directly to calculate
the range rect for a partial fragment selection. This makes the
selection rect visually stable - it no longer jitters when expanding the
selection character by character.
