Move ffi call into a method on png_image

rust/coffi/src/main.rs

        img.version = 1;
        return img
    }
    fn begin_read_from_file(&mut self, file_name: *const u8) -> u32 {
        unsafe { png_image_begin_read_from_file(self, file_name) as u32 }
    }
}
impl std::fmt::Display for png_image {
    let x = unsafe { cos(3.1415) };
    println!("cos(3.1415) = {}", x);
    unsafe {
        let mut img = png_image::new();
        println!("{}", img);
        let res = png_image_begin_read_from_file(&mut img, "mei.png\0".as_ptr());
        println!("read_from_file: {}", res);
        println!("{}", img);
    }
    let mut img = png_image::new();
    println!("{}", img);
    let res = img.begin_read_from_file("mei.png\0".as_ptr());
    println!("read_from_file: {}", res);
    println!("{}", img);
}