Fix an off-by-one bug when adding points to a diverging colormap

Symptom: if we added a new point on the lower side of the colormap,
adjacent to the middle "fixed point", then the new point would become
the fixed point and the original fixed point would become part of the
upper half of the colormap.

Author: njsmith

viscm/bezierbuilder.py

@@ -57,7 +57,7 @@ def get_control_points(self):
     def add_point(self, i, new_x, new_y):
         self._xp.insert(i, new_x)
         self._yp.insert(i, new_y)
-        if self._fixed is not None and i < self._fixed:
+        if self._fixed is not None and i <= self._fixed:
             self._fixed += 1
         self.trigger.fire()