Function imagequant_sys::liq_image_create_custom

#[no_mangle]
pub unsafe extern "C" fn liq_image_create_custom(
    attr: &liq_attr,
    row_callback: liq_image_get_rgba_row_callback,
    user_info: AnySyncSendPtr,
    width: c_uint,
    height: c_uint,
    gamma: f64
) -> Option<Box<liq_image<'static>>>