Fix odd frontline unit spawns

* Modfied frontline vector to ensure start point stays outside of
  exclusion zone.  (Previously it could be up to 100m inside)

* Change randomization in offset distance from frontline to be based
  on a percentage of the unit type's fixed offset instead of the
  width of frontline.
  (Prevents units from being far from their expected distance from
  frontline)

* Change visualgen to use the same frontline vector calculation as the
  unit spawns

game/utils.py

@@ -54,4 +54,4 @@ def heading_sum(h, a) -> int:
         return h
 
 def opposite_heading(h):
-    return h+180
+    return heading_sum(h, 180)