Fix for the Sinus Displacement geometry modifier not working properly.

The problem was caused by keeping a reference to a Vector object that is
assumed to be unchanged, but actually altered by the geometry modifier.

The same code updates were made to similar code portions to prevent
possible future bugs.

release/scripts/freestyle/style_modules/parameter_editor.py

@@ -174,11 +174,11 @@ def iter_t2d_along_stroke(stroke):
     total = stroke.length_2d
     distance = 0.0
     it = stroke.stroke_vertices_begin()
+    prev = it.object.point
     while not it.is_end:
         p = it.object.point
-        if not it.is_begin:
-            distance += (prev - p).length
-        prev = p
+        distance += (prev - p).length
+        prev = p.copy() # need a copy because the point can be altered
         t = min(distance / total, 1.0)
         yield it, t
         it.increment()
@@ -475,11 +475,11 @@ class CalligraphicThicknessShader(ThicknessBlenderMixIn, ScalarBlendModifier):
 def iter_distance_along_stroke(stroke):
     distance = 0.0
     it = stroke.stroke_vertices_begin()
+    prev = it.object.point
     while not it.is_end:
         p = it.object.point
-        if not it.is_begin:
-            distance += (prev - p).length
-        prev = p
+        distance += (prev - p).length
+        prev = p.copy() # need a copy because the point can be altered
         yield it, distance
         it.increment()
 
@@ -658,7 +658,7 @@ def iter_stroke_vertices(stroke):
         p = sv.point
         if prev_p is None or (prev_p - p).length > 1e-6:
             yield sv
-            prev_p = p
+            prev_p = p.copy()
         it.increment()
 
 class RoundCapShader(StrokeShader):