[qu2cuPen] Drop cubic implicit oncurves

Lib/fontTools/pens/qu2cuPen.py

@@ -73,4 +73,21 @@ class Qu2CuPen(ContourFilterPen):
             currentPt = args[-1] if args else None
         if quadratics:
             newContour.extend(self._quadratics_to_curve(quadratics))
+
+        # Add cubic implicit oncurve points
+        contour = newContour
+        newContour = []
+        for op, args in contour:
+            if (op == "curveTo" and newContour and newContour[-1][0] == "curveTo"):
+                pt0 = newContour[-1][1][-2]
+                pt1 = newContour[-1][1][-1]
+                pt2 = args[0]
+                if (pt2[0] - pt1[0] == pt1[0] - pt0[0] and
+                    pt2[1] - pt1[1] == pt1[1] - pt0[1]):
+                    newArgs = newContour[-1][1][:-1] + args
+                    newContour[-1] = ("curveTo", newArgs)
+                    continue
+
+            newContour.append((op, args))
+
         return newContour