Multiplayer-friendly shortcut improvements

Made some collision changes and fixed the checkpoint 3 shortcut to reduce the travel time between attempts

Updated preview image again

- Checkpoint 1 shortcut now only spawns when actually having reached the checkpoint
- Checkpoint 2 shortcut collision glitch fixed so it's no longer reachable too early
- Changed the level design around the checkpoint 3 shortcut so it's no longer reachable too early