Improve pathfinder support for multiple docks

The pathfinder can now find docks other than the Manhattan closest one.

Based on Cirdan commits:
afc7969c13d7ca59afe4dae4bf88122ba8d27df2
4067190dedcd3e5f668ea4f49b1dd8dfed10b2a7
d9be6b712d2ae4fb1b5ae844dde4919dd24c4fb2

src/dock_base.h

@@ -14,6 +14,7 @@
 
 #include "station_type.h"
 #include "tile_type.h"
+#include "map_func.h"
 #include "core/pool_type.hpp"
 
 typedef Pool<Dock, DockID, 32, 64000> DockPool;
@@ -31,6 +32,11 @@ struct Dock : DockPool::PoolItem<&_dock_pool> {
 
 	inline Dock *GetNextDock() const { return this->next; }
 
+	inline TileIndex GetDockingTile() const
+	{
+		return this->flat + TileOffsByDiagDir(DiagdirBetweenTiles(this->sloped, this->flat));
+	}
+
 	static Dock *GetByTile(TileIndex tile);
 };