Add a new prepareToRender() callback for renders to perform their initial window clears

This consolidates all the clearing that was strewn across the codebase.

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

@@ -13,6 +13,7 @@ public:
     virtual ~MmalRenderer() override;
     virtual bool initialize(PDECODER_PARAMETERS params) override;
     virtual bool prepareDecoderContext(AVCodecContext* context, AVDictionary** options) override;
+    virtual void prepareToRender() override;
     virtual void renderFrame(AVFrame* frame) override;
     virtual enum AVPixelFormat getPreferredPixelFormat(int videoFormat) override;
     virtual bool needsTestFrame() override;
@@ -23,8 +24,6 @@ private:
     static void InputPortCallback(MMAL_PORT_T* port, MMAL_BUFFER_HEADER_T* buffer);
     bool getDtDeviceStatus(QString name, bool ifUnknown);
     bool isMmalOverlaySupported();
-
-    void setupBackground(PDECODER_PARAMETERS params);
     void updateDisplayRegion();
 
     MMAL_COMPONENT_T* m_Renderer;