Function imagequant_sys::liq_image_set_background

pub unsafe extern "C" fn liq_image_set_background(
    img: &mut liq_image, 
    background: *mut liq_image
) -> liq_error

Remap assuming the image will be always presented exactly on top of this background.

Takes ownership of the background image (i.e. do NOT use it afterwards, do NOT call liq_image_destroy on it).

The background must have the same size. The foreground image must have a transparent color.