[core] reparse tiles when new data arrives

We're now reparsing tiles when they expire. We're also swapping out buckets atomically to avoid flickering data; i.e. we're displaying the old data as long as we don't have a new parsed bucket for that layer yet. The parsed buckets now live in the *TileData objects rather than in the TileWorker; only partially parsed == pending buckets will remain in the TileWorker. Once they're parsed, they're moved to the *TileData object.

src/mbgl/map/raster_tile_data.hpp

@@ -27,12 +27,13 @@ public:
     Bucket* getBucket(StyleLayer const &layer_desc) override;
 
 private:
+    TexturePool& texturePool;
     const SourceInfo& source;
     Worker& worker;
     RequestHolder req;
 
     RasterLayoutProperties layout;
-    RasterBucket bucket;
+    std::unique_ptr<Bucket> bucket;
 
     std::unique_ptr<WorkRequest> workRequest;
 };