0.5.1 - Tunnels

Small update.

• added the ability to buffer shifts for 0.25 seconds. this makes shifting more consistent if you hit the clutch slightly after hitting the shift input
• fixed a bug with author ghosts not appearing
• added an experimental tunnel asset to the Desert Speedway turn (finally, that audio reverb zone is useful for something)
• fixed a bug with hidden/unattainable wheels mistakenly appearing
• fixed timer bugs