Function bitmap 

pub fn bitmap(
    f: &TtFont,
    glyph: u32,
    size_fx: i32,
    png: Option<PngDecode>,
    scale: Option<ImgScale>,
    metrics: &mut [u8; 5],
) -> Option<Bitmap>

the colour bitmap of glyph at size_fx (24.8 px) [cbdt_bitmap]