Fix ceiling collision bug

When you un-duck or stop crawling, if there isn't sufficient headroom, you will go back to crawling.

Fixes #1594

Known issue: if you are forced into the crawling state like this, you will be stuck there until you press and release down with adequate space to stand. I can see about returning the player to standing automatically if you think it would be worth the time for me to do so, but I think it works well enough as is.