Multiplayer: Refactor to combine local and authoritative camera

[rainlizard]

Having a local camera which shadows the authoritative camera introduced hidden issues and I expect it will be even more troublesome in the future. So we're back to having one camera.

The original point of having the second camera was so we could have instant camera movements at high ping. But we can also achieve this by just not syncing any cameras in multiplayer while doing instant camera updates. However, not syncing cameras introduced two issues which had to be solved:

• The angle creatures are pushed when you slap them is informed by the player's current camera angle. Which is a problem if cameras aren't synced. To solve this I've turned Slap into a packet. Hopefully there aren't any slapping-related bugs because of this, but there could be.
• Packetsave/Packetload stopped having accurate cameras. It's been changed to directly save (and load) camera data every turn.

Also while I was adding the camera interpolation back, I made the first person camera deceleration a tad bit smoother. But it's pretty subtle.

Fixes: #4426 #4427 #4428

[rainlizard]

Closed in favour of camera lua flexibility: #4121