public boolean start(OutputStream os) {
        if (os == null)
            return false;
        boolean ok = true;
        closeStream = false;
        out = os;
        try {
            writeString("GIF89a"); // header
        } catch (IOException e) {
            ok = false;
        }
        return started = ok;
    }
Initiates GIF file creation on the given stream. The stream is not closed
automatically.

@param os
         OutputStream on which GIF images are written.
@return false if initial write failed.
core/src/main/java/com/bumptech/glide/gifencoder/AnimatedGifEncoder.java