Function collect_glyphs 

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

Clear glyphs and fill it with a GlyphInstance for every glyph in the given shaped_text_slices. Return a tuple that contains:

• The measure of the largest advance
• A boolean which is true if the slices only contained white space.