dgui: Fix DirectScrolledList scrollTo error in Python 3

As a rule of thumb, Python 3 divisions always have float results. Unfortunately, this piece of code is still relying on the old Python 2.7 behavior.

Closes #880

direct/src/gui/DirectScrolledList.py

@@ -210,7 +210,7 @@ class DirectScrolledList(DirectFrame):
         numItemsVisible = self["numItemsVisible"]
         numItemsTotal = len(self["items"])
         if(centered):
-            self.index = index - (numItemsVisible/2)
+            self.index = index - (numItemsVisible // 2)
         else:
             self.index = index