Rename png_image to PNGImage

To conform to the rust style of naming things.

rust/coffi/src/main.rs

}
#[repr(C)]
struct png_image {
struct PNGImage {
    opaque: *mut libc::c_void,
    version: libc::c_uint,
    width: libc::c_uint,
    message: [libc::c_char; 64],
}
impl png_image {
    fn new() -> png_image {
        let mut img: png_image = unsafe { std::mem::zeroed() };
impl PNGImage {
    fn new() -> PNGImage {
        let mut img: PNGImage = unsafe { std::mem::zeroed() };
        img.version = 1;
        return img
    }
    }
}
impl std::fmt::Display for png_image {
impl std::fmt::Display for PNGImage {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        fn get_message(msg: [libc::c_char; 64]) -> String {
            let mut vec = Vec::new();
#[link(name = "png")]
extern {
    fn png_image_begin_read_from_file(img: *mut png_image, file_name: *const libc::c_char) -> libc::c_int;
    fn png_image_begin_read_from_file(img: *mut PNGImage, file_name: *const libc::c_char) -> libc::c_int;
}
fn main() {
    println!("cos(3.1415) = {}", x);
    println!("");
    let mut img = png_image::new();
    let mut img = PNGImage::new();
    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());