don't show whole level, only revealed area

[KuSpa]

not all tiles are shown by default. only marked tiles in level spec.

whenever a wall tile becomes a ground tile (for whatever reason), reveal all neighbor tiles, that can be accessed now (recursive for neighbor ground tile).

extra: do a little animation, the further the distance, the later the tiles are revealed (just a fade from black to the new tile mesh?)

[KuSpa]

add concealed bool to tile struct, adapt mesh key in tile.rs concealed -> wall

[KuSpa]

lul

[KuSpa]

see #12