Function compile_chem_shapes 

pub fn compile_chem_shapes(
    glyphs: &[ChemGlyph],
    x: f64,
    y: f64,
    font_size: f64,
    anchor: &str,
    style: FaceStyle,
) -> Option<Shape>

Place a chem glyph run in SVG space (+Y down) with baseline at y.

anchor: start / middle / end on the advance box.