Fixes to demo

This is simply a fix as a result of the gracious feedback from friends!

• gravity is no longer tied to framerate! Play the game like the dev did!
• checkpoint! Feature not needed in demo, added from gamedev on my end.
• enemies are no longer damage immune upon reviving if they have been killed in a past player life.
• The chunkmother boss's hp now reset whenever the player respawn. They are back with a vengeance.