Add static points and deterministic physics

- Add static background. On each frame, there is a possibility that a
static point is promoted to a movable point, which then drops towards
the camera. If that happens, a new static point is created.
- Made the physics loop tick at constant 60 FPS, regardless of the
animation speed. This solves a problem where when the browser was
inactive, all stars would've been gone and an empty sky was shown for a
while.