Cache the result of GetImage() in Vehicle::UpdateViewport() where possible.

Sprite number is not cached if callback is made, or a variable access
outside a whitelist occurs.
Invalidate cached sprite number when direction or cargo changes,
or vehicle is marked dirty.

src/vehicle.cpp

@@ -367,6 +367,7 @@ Vehicle::Vehicle(VehicleType type)
 	this->cargo_age_counter  = 1;
 	this->last_station_visited = INVALID_STATION;
 	this->last_loading_station = INVALID_STATION;
+	this->cur_image_valid_dir  = INVALID_DIR;
 }
 
 /**