Rewrite FFmpeg decoder to use pull model

This allows us to keep asynchronous decoders like MMAL and V4L2M2M fed
while we're waiting for output frames. Behavior for synchronous decoders
should be identical.

Continuing to feed new data while waiting for output frames is crucial for
acceptable performance on 1080p video on the Raspberry Pi using V4L2M2M,
since it allows the decode and copy operations to be pipelined.

app/streaming/video/ffmpeg.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <functional>
+#include <QQueue>
 
 #include "decoder.h"
 #include "ffmpeg-renderers/renderer.h"
@@ -54,6 +55,10 @@ private:
     enum AVPixelFormat ffGetFormat(AVCodecContext* context,
                                    const enum AVPixelFormat* pixFmts);
 
+    void decoderThreadProc();
+
+    static int decoderThreadProcThunk(void* context);
+
     AVPacket* m_Pkt;
     AVCodecContext* m_VideoDecoderCtx;
     QByteArray m_DecodeBuffer;
@@ -74,11 +79,14 @@ private:
     int m_VideoFormat;
     bool m_NeedsSpsFixup;
     bool m_TestOnly;
-    enum {
-        RRF_UNKNOWN,
-        RRF_YES,
-        RRF_NO
-    } m_CanRetryReceiveFrame;
+    SDL_Thread* m_DecoderThread;
+    SDL_atomic_t m_DecoderThreadShouldQuit;
+
+    typedef struct {
+        uint64_t enqueueTimeMs;
+        uint32_t presentationTimeMs;
+    } FrameInfoTuple;
+    QQueue<FrameInfoTuple> m_FrameInfoQueue;
 
     static const uint8_t k_H264TestFrame[];
     static const uint8_t k_HEVCMainTestFrame[];