Convert Style to unique_ptr and pass by reference

Fixes #1277
Fixes #1309

src/mbgl/map/vector_tile_data.cpp

@@ -12,7 +12,7 @@ using namespace mbgl;
 
 VectorTileData::VectorTileData(const TileID& id_,
                                float mapMaxZoom,
-                               util::ptr<Style> style_,
+                               Style& style_,
                                GlyphAtlas& glyphAtlas_,
                                GlyphStore& glyphStore_,
                                SpriteAtlas& spriteAtlas_,
@@ -37,20 +37,12 @@ void VectorTileData::parse() {
     }
 
     try {
-        if (!style) {
-            throw std::runtime_error("style isn't present in VectorTileData object anymore");
-        }
-
         // Parsing creates state that is encapsulated in TileParser. While parsing,
         // the TileParser object writes results into this objects. All other state
         // is going to be discarded afterwards.
         VectorTile vectorTile(pbf((const uint8_t *)data.data(), data.size()));
         const VectorTile* vt = &vectorTile;
         TileParser parser(*vt, *this, style, glyphAtlas, glyphStore, spriteAtlas, sprite);
-
-        // Clear the style so that we don't have a cycle in the shared_ptr references.
-        style.reset();
-
         parser.parse();
     } catch (const std::exception& ex) {
         Log::Error(Event::ParseTile, "Parsing [%d/%d/%d] failed: %s", id.z, id.x, id.y, ex.what());