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/renderer.h

@@ -256,6 +256,12 @@ public:
         return false;
     }
 
+    virtual void prepareToRender() {
+        // Allow renderers to perform any final preparations for
+        // rendering after they have been selected to render. Such
+        // preparations might include clearing the window.
+    }
+
     // Allow renderers to expose their type
     enum class RendererType {
         Unknown,