Drop game mode

[cxong]

This branch implements the bare minimum required for a new game mode, "Drops" (similar to magical drop).

There is also a substantial refactor of game-mode-specific functions into a module with function pointer struct.

There are many more things to do to polish the game mode, including

• music
• background
• grid size
• block size
• animations
• special cursor
• sound effects
• adjusted speed settings
• flipped gravity and rise

[dorkster]

I've manually merged this. There were a lot of errors and warnings GCC reported that needed to be fixed.

One thing to add to the todo list: We should give a visual representation that the player has picked up and is holding blocks. Maybe rendering 1 block of the held type above the cursor would be enough?