support variable number of channels, png

[dllu]

Uses two bits to encode number of planes (1, 2, 3, or 4); implement reading png file. tested on monochrome and rgb png files.

For example to run on the Lossless Photo Compression Benchmark, we can download LPCB.7z from google drive and then run the following program:

extern crate hello_video_codec as hvc;
fn main() {
    let mut size = 0;
    let mut encode_time = std::time::Duration::new(0, 0);
    let mut decode_time = std::time::Duration::new(0, 0);
    for img in std::env::args().skip(1) {
        let frame = hvc::frame::RGB48Frame::from_png(&img).unwrap();

        let mut encoded = Vec::new();
        let now = std::time::Instant::now();
        frame.encode::<hvc::codec::Codec, _>(&mut encoded).unwrap();
        encode_time += now.elapsed();

        size += encoded.len();

        let now = std::time::Instant::now();
        let decoded = hvc::frame::RGB48Frame::decode::<hvc::codec::Codec, _>(
            &*encoded,
            frame.width,
            frame.height,
        )
        .unwrap();
        decode_time += now.elapsed();
        assert_eq!(frame == decoded, true);
        println!("{} : {} --> {}", &img, frame.data.len(), encoded.len());
    }
    println!("encode time: {} ms", encode_time.as_millis());
    println!("decode time: {} ms", decode_time.as_millis());
    println!("size: {} bytes", size);
}

We note that LPCB.7z contains a monochrome PNG file

identify /Users/dllu/Downloads/LPCB/PIA13943.png
/Users/dllu/Downloads/LPCB/PIA13943.png PNG 3000x2400 3000x2400+0+0 8-bit Gray 256c 560526B 0.000u 0:00.000