Added support for clearing map graph when map is disposed

Disposing maps was leaving invalid pathfinding results -- this fixes that

core/src/com/riiablo/map/Map.java

@@ -365,6 +365,7 @@ public class Map implements Disposable {
     zones.clear();
     for (DT1s dt1s : this.dt1s.values()) dt1s.clear();
     dt1s.clear();
+    mapGraph.clear();
   }
 
   public Array<AssetDescriptor> getDependencies() {

core/src/com/riiablo/map/MapGraph.java

@@ -24,6 +24,10 @@ public class MapGraph {
     smoother = new PathSmoother<>(raycaster);
   }
 
+  public void clear() {
+    identity.clear();
+  }
+
   public Point2 getOrCreate(Vector2 src) {
     return getOrCreate(tmpPoint.set(src));
   }