Camera tracking: added depth object to Follow Track constraint

If this object is defined, object with Follow Track constraint would be
projected into surface of this depth object.
If object is not set or there's no projection onto it, projection plane
calculated based on original object position would be used.

This allows to make cheap facial mocap.

release/scripts/startup/bl_ui/properties_object_constraint.py

@@ -778,6 +778,10 @@ class ConstraintButtonsPanel():
 
         layout.prop(con, "camera")
 
+        row = layout.row()
+        row.active = not con.use_3d_position
+        row.prop(con, "depth_object")
+
         layout.operator("clip.constraint_to_fcurve")
 
     def CAMERA_SOLVER(self, context, layout, con):