Tie bit rates tested to compression level

[nfrechette]

By controlling how many bit rates we attempt, we can control how long compression takes and how small the memory footprint will be. Once we have the full quaternion variable bit rate introduced, the compression time will increase significantly. An enum needs to be introduced in the compression settings to control how aggressive we should be: Low, Medium, High, Ultimate or something like that.

Every level should be able to reach the error threshold by always allowing the highest bit rate but lower levels might be dropped according to some rule: 25%, 50%, 75% dropped maybe. Sensible values should be picked.