Function reachability_floodfill 

pub fn reachability_floodfill(
    origins: &Vec<RoomXY>,
    obstacles: &LocalCostMatrix,
) -> LocalCostMatrix

Generates a reachability floodfill from origin positions.

Takes a Vec of origin locations to start the floodfill from, and a Cost Matrix of obstacles, and produces a Cost Matrix with 1 values for all positions that can be reached from the origin points, and 0 values everywhere else.

The obstacles Cost Matrix should have u8::MAX set on all positions that are obstacles, and 0 everywhere else.