Keep players from falling through moving frames

Before, when a frame moved, it removed the whole thing, then reappeared in its new location. If it takes any time at all to do this, a player will fall while the machine doesn't exist, then be stuck inside the machine or completely under it when it reappears.
Now, when a frame moves, it creates a copy in its new location, then removes any part of the original that wasn't overwritten by the copy. This way, the machine still exists, and a wave of duplication goes through it, followed by a wave of deletion.

Here's a textual demonstration:

P represents the player, other letters represent frames and nodes moved by frames, and - represents the ground.

```
Before:

----
 P
ABCD

----

 P
ABC

----
 P
AB

----
 P
A

----

 P

----

    D
 P
----

   CD
 P
----

  BCD
 P
----

 ABCD
 P
----
```

```
After:

----
 P
ABCD

----
 P
ABCDD

----
 P
ABCCD

----
 P
ABBCD

----
 P
AABCD

----
 P
 ABCD

----