Add a paused flag in addition to the running flag.
Implement game loop pausing.
Re-export new procedures from game-loop module.
Move game-fps variable.
Add error handling in game loop.

Print backtrace and pause game when exceptions occur. This way the
developer can fix the error and resume the game rather than the game
crashing, forcing the developer to restart the game. The REPL remains
active when the game is paused, of course.
Simplify scene stack.