Text alignment
Use gray instead of black as sprite alpha

This lets us use both black and white pixels in sprite source, and
keep both easily visible.
Sprite work
Update to Rust 0.10-pre 5b4a141
Different stair down tile

Had a vague idea to put this under the normal floor plane when it's
going downstairs, but I'm not sure how much that will end up making
sense with the hex floors.
Sprite work
Square and hex walls