Support getting the filename from the cmdline

rust/coffi/src/main.rs

    flags: libc::uint32_t,
    colormap_entries: libc::uint32_t,
    warning_or_error:  libc::uint32_t,
    message: [u8; 64],
    message: [libc::c_char; 64],
}
impl png_image {
        return img
    }
    fn begin_read_from_file(&mut self, file_name: *const u8) -> u32 {
    fn begin_read_from_file(&mut self, file_name: *const libc::c_char) -> u32 {
        unsafe { png_image_begin_read_from_file(self, file_name) as u32 }
    }
}
impl std::fmt::Display for png_image {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        fn get_message(msg: [u8; 64]) -> String {
        fn get_message(msg: [libc::c_char; 64]) -> String {
            let mut vec = Vec::new();
            for i in 0..64 {
                vec.push(msg[i]);
                vec.push(msg[i] as u8);
            }
            String::from_utf8(vec).unwrap()
        }
#[link(name = "png")]
extern {
    fn png_image_begin_read_from_file(img: *mut png_image, file_name: *const u8) -> libc::c_int;
    fn png_image_begin_read_from_file(img: *mut png_image, file_name: *const libc::c_char) -> libc::c_int;
}
fn main() {
    let mut img = png_image::new();
    println!("{}", img);
    let res = img.begin_read_from_file("mei.png\0".as_ptr());
    let file_name = std::env::args().nth(1).unwrap_or(String::from("mei.png"));
    let c_name = std::ffi::CString::new(file_name).unwrap();
    let res = img.begin_read_from_file(c_name.as_ptr());
    println!("read_from_file: {}", res);
    println!("{}", img);
}