Adds checkpoint creation to GameManager.

Adds a CheckpointBox prefab (derived from JetstreamBox) and triggers
generation along the river after GameManager receives the river's
points list. Size is currently hardcoded, but maybe should raycast
to the shore to determine width?

Checkpoints don't currently do anything. They will inform GameManager
or RaceManager about relative player positions.
Fixes input bugs for 2-player operation.