According to the CSS font matching algorithm specification, it is
supposed to be executed for each glyph instead of each text run, as is
currently done. This change partially implements this by having the
font matching algorithm produce a list of fonts against which each
glyph will be tested to find its suitable font.
Now, it becomes possible to have per-glyph fallback fonts: if the
needed glyph is not present in a font, we can check the subsequent
fonts in the list.
Add the CanvasTextDrawingStyles mixin with the textAlign and
textBaseline attributes. Update fill_text in CanvasRenderingContext2D
to move the text rect by the text align and text baseline attributes.
Wrote a simple HTML example to showcase the new features.
