Clear default framebuffer when drawing from our rtt for first cam

Also remove unused IReferenceCounted in post processing

src/graphics/post_processing.cpp

@@ -742,7 +742,7 @@ public:
 
 // ============================================================================
 
-PostProcessing::PostProcessing(IVideoDriver* video_driver)
+PostProcessing::PostProcessing()
 {
     // Load the MLAA area map
     io::IReadFile *areamap = irr_driver->getDevice()->getFileSystem()->
@@ -760,12 +760,6 @@ PostProcessing::PostProcessing(IVideoDriver* video_driver)
 
 }   // PostProcessing
 
-// ----------------------------------------------------------------------------
-PostProcessing::~PostProcessing()
-{
-    // TODO: do we have to delete/drop anything?
-}   // ~PostProcessing
-
 // ----------------------------------------------------------------------------
 /** Initialises post processing at the (re-)start of a race. This sets up
  *  the vertices, normals and texture coordinates for each