Make the state attribute private

Also add a virtual setState() method to give subclasses a chance
to intercept the change of state. This will be used by VectorTileData
to release resources when the TileData becomes immutable.

src/mbgl/map/tile_cache.cpp

@@ -46,7 +46,7 @@ std::shared_ptr<TileData> TileCache::get(uint64_t key) {
         data = it->second;
         tiles.erase(it);
         orderedKeys.remove(key);
-        assert(data->ready());
+        assert(data->isReady());
     }
 
     return data;