[varStore] Another fix and test

Lib/fontTools/varLib/varStore.py

@@ -382,7 +382,7 @@ class _Encoding(object):
         """Maximum possible byte gain from merging this into another
         characteristic."""
         count = len(self.items)
-        return max(0, self.overhead - count * (self.width + 1))
+        return max(0, self.overhead - count)
 
     def sort_key(self):
         return self.width, self.chars

Tests/varLib/varStore_test.py

@@ -135,8 +135,29 @@ def buildAxis(axisTag):
                 [10, 303, 0, 0, 20],
                 [10, 304, 0, 0, 20],
             ],
+            1,
+            200,
+        ),
+        (
+            5,
+            [
+                [0, 11, 12, 0, 20],
+                [0, 13, 12, 0, 20],
+                [0, 14, 12, 0, 20],
+                [0, 15, 12, 0, 20],
+                [0, 16, 12, 0, 20],
+                [0, 17, 12, 0, 20],
+                [0, 18, 12, 0, 20],
+                [0, 19, 12, 0, 20],
+                [0, 20, 12, 0, 20],
+                [10, 300, 0, 0, 20],
+                [10, 301, 0, 0, 20],
+                [10, 302, 0, 0, 20],
+                [10, 303, 0, 0, 20],
+                [10, 304, 0, 0, 20],
+            ],
             2,
-            206,
+            218,
         ),
         (
             3,