Function composite 

pub fn composite(
    dst: &mut [u8],
    dst_w: u32,
    dst_h: u32,
    tile: &CoverageTile,
    x: i32,
    y: i32,
    color: [u8; 4],
)

Composite an A8 coverage tile at (x, y) in color (see the private composite_coverage helper).