Decouple input timing sensitivity from rendering

[Ralith]

We currently only collect input between frames. This makes the precision of control over movement (for example) proportional to framerate. We could improve on this by relying on precise input timestamps. Ideally we'd extend winit to collect these from the OS, but a more tractible approach would be to punt rendering/simulation onto a separate thread from input, record our own timestamps in the input thread, and stream timestamped events to the simulation.