filter: Remove the silly dependency on the ShowBase instance

Closes #1302

direct/src/filter/FilterManager.py

@@ -322,7 +322,7 @@ class FilterManager(DirectObject):
             props.setAuxRgba(1)
         if (auxtex1 != None):
             props.setAuxRgba(2)
-        buffer=base.graphicsEngine.makeOutput(
+        buffer=self.engine.makeOutput(
             self.win.getPipe(), name, -1,
             props, winprops, GraphicsPipe.BFRefuseWindow | GraphicsPipe.BFResizeable,
             self.win.getGsg(), self.win)