Fixed a few tile placement bugs! And other aesthetics

Alpha 2!

Nothing much to discuss! Added code prevent players from walling themselves into a cliff or drowning themselves. Also prevented crops from being unearthed, and just overall refined the tile placement to not allow things to overlap that shouldn't... like cliffs on top of water, for example...

Added a background! Made the Day/Night cycle more snappy! The background is very tentative, just wanted to get a feel how well I could integrate a freedrawn asset with a tile-based grid. There's... a lot of layering and colliders involved, but it seems doable!

Still haven't fixed the ramps, but I've got a good idea on how to get it done with custom data layers.