use IDs for multiple decoders

app/decoder/ffmpeg/ffmpegdecoder.h

@@ -37,14 +37,20 @@ extern "C" {
 class FFmpegDecoder : public Decoder
 {
 public:
+  // Constructor
   FFmpegDecoder();
 
+  // Destructor
+  virtual ~FFmpegDecoder() override;
+
   virtual bool Probe(Footage *f) override;
 
   virtual bool Open() override;
   virtual FramePtr Retrieve(const rational &timecode, const rational &length = 0) override;
   virtual void Close() override;
 
+  virtual QString id() override;
+
 private:
   /**
    * @brief Handle an error
@@ -118,6 +124,8 @@ private:
    */
   AVPixelFormat GetCompatiblePixelFormat(const AVPixelFormat& pix_fmt);
 
+  int64_t GetClosestTimestampInIndex(const int64_t& ts);
+
   AVFormatContext* fmt_ctx_;
   AVCodecContext* codec_ctx_;
   AVStream* avstream_;