implemented core subtitle support

app/codec/ffmpeg/ffmpegencoder.h

@@ -46,6 +46,8 @@ public:
 
   virtual bool WriteAudio(olive::SampleBufferPtr audio) override;
 
+  virtual bool WriteSubtitle(const SubtitleBlock *sub_block) override;
+
   virtual void Close() override;
 
   virtual VideoParams::Format GetDesiredPixelFormat() const override
@@ -90,6 +92,9 @@ private:
   int audio_frame_offset_;
   int audio_write_count_;
 
+  AVStream* subtitle_stream_;
+  AVCodecContext* subtitle_codec_ctx_;
+
   bool open_;
 
 };