prevent a nasty gl/panda issue with buffers getting thrown off the card

direct/src/showbase/Transitions.py

@@ -135,6 +135,7 @@ class Transitions:
         if gsg:
             # If we're about to fade in from black, go ahead and
             # preload all the textures etc.
+            base.graphicsEngine.renderFrame()
             render.prepareScene(gsg)
             render2d.prepareScene(gsg)