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_parser.cpp

@@ -41,7 +41,9 @@ TileParser::TileParser(const GeometryTile& geometryTile_,
     assert(collision);
 }
 
-bool TileParser::obsolete() const { return tile.state == TileData::State::obsolete; }
+bool TileParser::obsolete() const {
+    return tile.getState() == TileData::State::obsolete;
+}
 
 void TileParser::parse() {
     for (const auto& layer_desc : style.layers) {