Core/Maps: Improvements to terrain swap handling

* Fixed memory leak when unloading grids
* Handle child maps being entered
* Allow chaining more child maps (Draenor -> Tanaan Jungle -> Tanaan Jungle - No Hubs Phase)

src/common/Collision/Maps/MapTree.cpp

@@ -242,10 +242,14 @@ namespace VMAP
         if (!result.File)
         {
             int32 parentMapId = vm->getParentMapId(mapID);
-            if (parentMapId != -1)
+            while (parentMapId != -1)
             {
                 result.Name = basePath + getTileFileName(parentMapId, tileX, tileY);
                 result.File = fopen(result.Name.c_str(), "rb");
+                if (result.File)
+                    break;
+
+                parentMapId = vm->getParentMapId(uint32(parentMapId));
             }
         }