Use channel area for overview image

This allows unprojected data to have correct geolocation info.

mpop/instruments/visir.py

@@ -152,7 +152,7 @@ def overview_sun(self, stretch='linear', gamma=1.6, fill_value=(0, 0, 0)):
         blue = -self[10.8].data
 
         img = geo_image.GeoImage((red, green, blue),
-                                 self.area,
+                                 self[10.8].area,
                                  self.time_slot,
                                  fill_value=fill_value,
                                  mode="RGB")