layout: Implement text-align: justify and text-justify per

CSS-TEXT-3 § 7.3.

`text-justify: distribute` is not supported.

The behavior of `text-justify: none` does not seem to match what Firefox
and Chrome do, but it seems to match the spec.

Closes #213.

components/gfx/text/shaping/harfbuzz.rs

@@ -464,7 +464,7 @@ impl Shaper {
                                           false,
                                           true,
                                           true);
-                glyphs.add_glyph_for_char_index(char_idx, &data);
+                glyphs.add_glyph_for_char_index(char_idx, Some(character), &data);
             } else {
                 // collect all glyphs to be assigned to the first character.
                 let mut datas = vec!();