Viewport: Cache landscape pixels in map mode

Avoid invalidating landscape pixels for non-landscape updates
(vehicles, overlays, etc.)

src/town_gui.cpp

@@ -275,7 +275,7 @@ public:
 
 				this->town->show_zone = new_show_state;
 				this->SetWidgetLoweredState(widget, new_show_state);
-				MarkWholeScreenDirty();
+				MarkWholeNonMapViewportsDirty();
 				break;
 			}