Add in new images for existing tiles.

NB: These images are horrible misleading in this commit! The tiles still
only have one exit per edge, despite appearances.
Makes tiles have two possible exits per side
Rename Tile::Corner -> Tile::LooseCorner
Only use blank tiles for obstacles
Add Tile::TightCorner
Base water flow time on pipe length
Add Tile::WaveLeft and Tile::WaveRight
Add Tile::HookLeft and Tile::HookRight
New tileset with two possible exits per side