directtools: Fix repeated selections causing scaling node to get huge

This appears to be a regression from 0fe56bd0a980e0791d129498e2eb60ade6a0506d, but I can't be sure.  Before this fix, repeated clicks of an object would cause the scaling handles to get larger and larger, until eventually causing NaN assertions.

direct/src/directtools/DirectManipulation.py

@@ -1368,7 +1368,7 @@ class ObjectHandles(NodePath, DirectObject):
         self.setScalingFactor(1)
 
     def setScalingFactor(self, scaleFactor):
-        self.ohScalingFactor = self.ohScalingFactor * scaleFactor
+        self.ohScalingFactor = scaleFactor
         self.scalingNode.setScale(self.ohScalingFactor * self.directScalingFactor)
 
     def getScalingFactor(self):