Fix race condition where a separate test decoder could ingest a real frame

app/streaming/video/ffmpeg.h

@@ -33,10 +33,21 @@ public:
     virtual IFFmpegRenderer* getBackendRenderer();
 
 private:
+    enum class TestMode {
+        // No test frame and prepare for rendering
+        NoTesting,
+
+        // Submit only the test frame and do not prepare for rendering
+        TestFrameOnly,
+
+        // Submit the test frame and prepare for rendering
+        TestFrame
+    };
+
     bool completeInitialization(const AVCodec* decoder,
                                 enum AVPixelFormat requiredFormat,
                                 PDECODER_PARAMETERS params,
-                                bool testFrame,
+                                TestMode testMode,
                                 bool useAlternateFrontend);
 
     void stringifyVideoStats(VIDEO_STATS& stats, char* output, int length);
@@ -116,6 +127,7 @@ private:
     int m_VideoFormat;
     bool m_NeedsSpsFixup;
     bool m_TestOnly;
+    TestMode m_CurrentTestMode;
     SDL_Thread* m_DecoderThread;
     SDL_atomic_t m_DecoderThreadShouldQuit;