Adaptive interpolation (more responsive movement for players and NPCs)

[miredirex]

Other players and NPCs now move more smoothly and sit much closer to real time.

Before: remote actors were always shown a fixed 0.3s in the past (regardless of actual ping), so they felt laggy. Network hiccups could also make them freeze for a few seconds.

In action, before vs after on localhost (notice the delay):

combined.webm

Simulator that was used to test different network conditions and latency: https://gist.github.com/miredirex/710941b6b75f44d36f463755d55eb478

Player-friendly changelog entry: Improved movement sync: other players and NPCs now move more smoothly and with much less delay

---

Note on bandwidth:

If bandwidth ever becomes a problem, the easiest savings are:

1. Delta-encode the animation variables against the last snapshot. Most of the ~120 bytes are animation floats that hardly change between frames. Because movement is unreliable, this needs either acks for which snapshot the other side has, or a full keyframe every N snapshots. That could easily halve the size.
2. Lower the send rate for actors that are far away from the receiving player, not just for idle ones.
3. Skip snapshots for actors that haven't moved. Standing NPCs currently still send 10 times a second.