Added GraphicsContext support for clearing the graphis context. The clearing

is off by default (ClearMask is 0), but can be switched on when ClearMask is
non zero.  GraphicsContext::clear() is called after each swap buffers

src/osg/GraphicsThread.cpp

@@ -379,6 +379,7 @@ void SwapBuffersOperation::operator () (GraphicsContext* context)
     if (context)
     {
         context->swapBuffersImplementation();
+        context->clear();
     }
 }