ffmpeg: disable multithreading in favor of our own

Our renderer system is already heavily multithreaded and prioritized to keep the
main thread active. As a result, FFmpeg receives little benefits from being
multithreaded and is actually detrimental to our main thread as it hasn't been
prioritized to keep the main thread as active as possible.

app/codec/ffmpeg/ffmpegdecoder.h

@@ -61,6 +61,8 @@ public:
   virtual bool SupportsVideo() override;
   virtual bool SupportsAudio() override;
 
+  void SetMultithreading(bool e);
+
 private:
   void ConformInternal(SwrContext *resampler, WaveOutput *output, const char *in_data, int in_sample_count);
 
@@ -145,6 +147,8 @@ private:
 
   QVector<int64_t> frame_index_;
 
+  bool multithreading_;
+
 };
 
 #endif // FFMPEGDECODER_H