More realistic AI behavior

A small update that makes the AI behave more convincing.

They tended to be a little 'blocky', needlessly braking on straights a tiny bit, which felt frustrating and unrealistic to race against.

Also, they crashed too much on higher drift style levels, and on slippery tracks. This has also been improved.

Keep in mind this results in the AI being generally faster - there's an AI difficulty slider in the gameplay settings if it became too difficult!