oops, don't ask for a back buffer

direct/src/filter/FilterManager.py

@@ -295,6 +295,7 @@ class FilterManager(DirectObject):
         winprops = WindowProperties()
         winprops.setSize(xsize, ysize)
         props = FrameBufferProperties(self.win.getFbProperties())
+        props.setBackBuffers(0)
         props.setRgbColor(1)
         props.setDepthBits(depthbits)
         depthtex, colortex, auxtex0, auxtex1 = texgroup