Function glyphs 

fn glyphs(
    shaped_text_slices: &[Arc<ShapedTextSlice>],
    baseline_origin: PhysicalPoint<Au>,
    justification_adjustment: Au,
) -> (Vec<GlyphInstance>, Au, bool)

Return a tuple for the given shaped_text_slices that contains:

• A vector of GlyphInstance for every glyph in the slices.
• The measure of the largest advance
• A boolean which is true if the slices only contained white space.