Add support for multiple tire sizes

[agateau]

To add more variety to the game vehicles, I want to be able to define the tire size per axle.

This issue tracks the required game engine changes to support this.

• [x] Load tire size from the vehicle XML file
• [x] Draw the tire shadows (vehicles with big tires look odd without tire shadows)
• [ ] Adjust size of water splashes to the tire size
• [x] Always draw tires, even when vehicle is drowning
• [x] Fix weird tire sizing issue when vehicle is lifted by an helicopter
• [ ] Adjust the tire body density to ensure vehicles with big tires do not feel too heavy