Add a VsyncSource for renderers that already block for V-sync

app/streaming/video/ffmpeg-renderers/pacer/nullthreadedvsyncsource.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "pacer.h"
+
+class NullThreadedVsyncSource : public IVsyncSource
+{
+public:
+    NullThreadedVsyncSource(Pacer* pacer);
+
+    virtual ~NullThreadedVsyncSource();
+
+    virtual bool initialize(SDL_Window* window);
+
+private:
+    static int vsyncThread(void* context);
+
+    Pacer* m_Pacer;
+    SDL_Thread* m_Thread;
+    SDL_atomic_t m_Stopping;
+};