Improved vehicle physics in my prototype

In today's update, I've made the cars in my game more realistic by giving them brakes and improving the way the tire forces are simulated. Previously, the slip for the longitudinal and lateral movement of the tire was treated as separate values, but I've now combined them into a single vector with a maximum magnitude of 1. This allows for the slip to be larger than the maximum force that is applied, which means that the car can now drift/slide. Although drifting is currently difficult to do, I plan to add more logic to the car itself in future updates to make it easier.