Separate FFmpeg decoder from the Session class (#4)

app/streaming/video/ffmpeg-renderers/vt.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include "renderer.h"
+
+// A factory is required to avoid pulling in
+// incompatible Objective-C headers.
+class VTRendererFactory {
+public:
+    static
+    IFFmpegRenderer* createRenderer();
+};