don't drop implied oncurves on interpolatable TTFs

this should only be done at the end in the final VF, otherwise masters end up with incompatible glyphs...

Lib/ufo2ft/_compilers/interpolatableTTFCompiler.py

@@ -75,6 +75,9 @@ def compileOutlines(self, ufo, glyphSet, layerName=None):
         # can compute impliable on-curve points from unrounded coordinates before
         # building the VF
         kwargs["roundCoordinates"] = False
+        # keep impliable oncurve points in the interpolatable master TTFs, they will
+        # be pruned at the end by varLib the final VF.
+        kwargs["dropImpliedOnCurves"] = False
         outlineCompiler = self.outlineCompilerClass(ufo, glyphSet=glyphSet, **kwargs)
         return outlineCompiler.compile()