Player re-spawn when falling off the stage

[nicholaschuayunzhi]

Currently the player must press esc to respawn after falling off the stage.

Let's add a mechanism that automatically re spawns the player at the checkpoint.

Two ways to do it:

1. Restart the level as per normal
2. (Suggestion from GDG member) Player keeps falling and suddenly lands on the checkpoint
   • lose the spawn in transition