Use None as centre_point sentinel value instead of [-1, -1]

[Frimkron]

If centre_point is [-1, -1], Myrmidon uses the centre of the graphic instead, but this prevents the user from actually using [-1, -1] as the centre point. If None was used to denote this preference instead, this would be less confusing.

[Fiona]

don't tell me what to do mom

[Frimkron]

It actually treats [-1, x] and [x, -1] as sentinels too