Image: [Suggestion] WebP Encoding Support

[liftoff]

The crate currently supports WebP decoding but it would be nice if it also supported encoding as well. I'd definitely take advantage of it in one of my forthcoming projects.

[liftoff]

For reference purposes (in case anyone wants to know how to encode WebP in the mean time), here's how to encode an ImageBuffer to WebP via libwebp/libc:

extern crate libc;
use libc::{c_int, c_float, size_t};
use std::io::{Write, BufWriter};
use std::fs::File;
use std::path::Path;

#[link(name = "webp")]
extern {
// Here's the C lib's function def:
//     size_t WebPEncodeRGB(const uint8_t* rgb, int width, int height, int stride, float quality_factor, uint8_t** output);
//     size_t WebPEncodeLosslessRGB(const uint8_t* rgb, int width, int height, int stride, uint8_t** output);

    fn WebPEncodeRGB(rgb: *const u8, width: c_int, height: c_int, stride: c_int, quality_factor: c_float, output: *mut *mut u8) -> size_t;
    fn WebPEncodeLosslessRGB(rgb: *const u8, width: c_int, height: c_int, stride: c_int, output: *mut *mut u8) -> size_t;
    fn WebPFree(ptr: *const u8);
}

let filetype = "webp";
let ref path_str = format!("/tmp/test.{}", filetype);
let path = &Path::new(path_str);
// Pretend this screenshot() function returns ImageBuffer<Rgb<u8>, Vec<u8>>:
let img = screenshot();
let stride = width * 3;
let lossless = false; // Set to true for lossless (Warning: CPU intensive/slow)
let quality: c_float = 75.0; // Quality level of the WebP image
let mut output: *mut u8 = std::ptr::null_mut();
let raw = img.clone().into_raw();
unsafe {
    let length: usize;
    if lossless {
        length = WebPEncodeLosslessRGB(raw.as_ptr(), width as c_int, height as c_int, stride as c_int, &mut output);
    } else {
        length = WebPEncodeRGB(raw.as_ptr(), width as c_int, height as c_int, stride as c_int, quality, &mut output);
    }
    let mut fout = BufWriter::new(File::create(&path).unwrap());
    let slice = std::slice::from_raw_parts(output, length);
    let _ = fout.write_all(slice).unwrap();
    let _ = fout.flush();
    let _ = WebPFree(output); // IMPORTANT: Make sure we free that memory
}