Mounting creature while inside too-small corridor bypasses size restriction

[Frumple]

As per title. If you enter a corridor that is too small to enter while mounted, then mount your ride from that square, you'll be able to ride through the corridor unhindered.

With the attached character, you can observe that the corridor directly to the south restricts entrance, then dismount, step one square south, and remount (you may have to order the nighthunter to travel near, to get it close enough) inside the corridor. Afterwards leaving the corridor and trying to re-enter while mounted, naturally enough, prevents your entrance.

[RMTEW FULL NAME]

Prevent mounting on a tile where it would not be possible to move to when mounted anyway. Reported by Frumple. Closes #109

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