Change the neighbours vector to a set in AStar

This fixes an issue where one could not disconnect two points that were connected more than once.

core/math/a_star.h

@@ -54,7 +54,7 @@ class AStar : public Reference {
 		real_t weight_scale;
 		uint64_t last_pass;
 
-		Vector<Point *> neighbours;
+		Set<Point *> neighbours;
 
 		// Used for pathfinding
 		Point *prev_point;