- extracted createGlassOverlay() function for its 3 uses
- car movement now uses simulated physics with acceleration taking into account current velocity, engine power and friction (using 2D vector maths)
- one-way logic simplified by executing a callback method when collision with OneWay component starts AND ends  (previously just on start)
- no longer having different sound when car speeds up, so removed engine_rev_faster.ogg
- less shaking of whole screen when movement repeatedly stopped due to multiple successive collisions by setting velocity to zero
- fixed bug in Countdown which caused time to jump up after pause and unpause