Add a direction to the list of valid directions for a given coordinate.
Get the internal value for a given coordinate.
Get the list of valid directions for a given coordinate.
Set the internal value for a given coordinate.
Set the list of valid directions for a given coordinate.
A flow field is a 50x50 grid (representing a room), representing viable directions to travel to reach a particular target (or targets). A given tile may have multiple equally valid directions, so we represent this as a bitfield (where each bit in an 8-bit unsigned integer represents a direction that is either viable or not).