Adds test scene, greatly improves buoyancy and controls.

Adds a test scene with a river, a boat, and a follow cam.

Improves boat buoyancy by applying forces to outriggers away from
boat's center of mass. Adjusts forces and float height to match.

Improves control by applying force to counteract lateral slide.

Improves control by using half-circles for rudder input instead of
full circles.

Refactors joystick swirl detection to make sure a swirl is aborted
if there's an improper input. Old code didn't have cancels, so every
fourth clockwise swirl would register one counter-clockwise swirl.
Pattern counter advanced whenever the next input appeared within the
timeout window regardless of inputs received in the meantime.

Fixes rudder linkage to boat control script.

Adds MiniJSON.cs and begins to add river seed data export to JSON.