More Knockback

Changes
• Gave player longspear to start, for testing knockback more easily.
• Monsters no longer get knocked back into solid props that report
their position to the world variable.
• Monsters can have different “knockback amounts” specified, so
lighter/weaker things get knocked back more than other monsters.
• New function for getting a grid tile based on X,Y coordinates, with
options for rounding or flooring.
• Some better comments.
• Only knock back a monster if it wouldn’t die—this fixes a lot of bugs.
Known Bugs
• No new ones… yay?
• Though, I really want to make my knockback function simpler/cleaner.