Gates that close when you go through them

[thejoshwolfe]

First implemented by @Patashu in #24.

After each gravity tick, if an OpenGate was occupied and now isn't, it becomes a solid ClosedGate.

(The list of occupied OpenGates is added to before and after initial movement, and after portals activate.)

This is different to the behaviour of portal activation - for example, you can push different objects past an OpenGate without it closing, and can even pull a snake tail away from an OpenGate, but have an object fall on it in the first gravity tick, without it closing. (If it takes more than one gravity tick, it will close.)