map: add set_colour on a polygon

Author: peterbarker

Diff for: MAVProxy/modules/mavproxy_map/mp_slipmap_util.py

@@ -226,6 +226,9 @@ def __init__(self, key, points, layer, colour, linewidth, arrow = False, popup_m
         self._has_timestamps = False
         self._showlines = showlines
 
+    def set_colour(self, colour):
+        self.colour = colour
+
     def bounds(self):
         '''return bounding box'''
         if self.hidden: