Add option to path using "whitelist" instead of "blacklist"

[darzu]

I think it’d be neat if pathing could specify a set of tiles types that sprites could use (e.g. road tiles), instead of just “anything but walls”.

Essentially a “whitelist” of valid tiles (e.g. roads) instead of a “blacklist” of tiles to avoid (e.g. walls).

This decoupling with walls could be nice, because walls are used for many reasons. In one of my games I temporarily set a bunch of a walls, do the pathing, then unset those wall locations just so I can get pathing to work how I want.

[eanders-ms]

Just noticed this issue after making this: #14. It lets you provide an optional pathable tile image. A list would be better, but makes the block more complicated. Maybe break it up in to a few different blocks of varying complexity?