Tracking: Implement Nuke/Natron distortion model

Neither Nuke nor Natron support OpenCV's radial distortion model
which makes it impossible to have any kind of interoperability.

The new model is available under the distortion model menu in Lens
settings.

Differential Revision: https://developer.blender.org/D7484

release/scripts/startup/bl_ui/space_clip.py

@@ -918,6 +918,10 @@ class CLIP_PT_tracking_lens(Panel):
             col = layout.column(align=True)
             col.prop(camera, "division_k1")
             col.prop(camera, "division_k2")
+        elif camera.distortion_model == 'NUKE':
+            col = layout.column(align=True)
+            col.prop(camera, "nuke_k1")
+            col.prop(camera, "nuke_k2")
 
 
 class CLIP_PT_marker(CLIP_PT_tracking_panel, Panel):