Fixes #560: add weighted A* pathfinding with friction surface

Replace the O(h*w) open-list scan with a binary min-heap (reused from
cost_distance) and add optional friction weighting to a_star_search().
Edge costs now use cellsize-scaled spatial distances instead of pixel-
space distances, and when a friction surface is provided the cost model
matches cost_distance (geometric_distance * mean_friction_of_endpoints).

Author: brendancol