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/ground_vehicle.cpp

@@ -108,6 +108,7 @@ void GroundVehicle<T, Type>::CargoChanged()
 		weight += current_weight;
 		/* Slope steepness is in percent, result in N. */
 		u->gcache.cached_slope_resistance = current_weight * u->GetSlopeSteepness() * 100;
+		u->cur_image_valid_dir = INVALID_DIR;
 	}
 
 	/* Store consist weight in cache. */