Simplify RunLoop

Instead of transforming between return value and callback, pass
the wrapped callback on to the invoked function as the last argument.

This eliminates the need for multiple invokeWithArgument overloads
and permits invoked functions to be asynchronous.

src/mbgl/map/source.cpp

@@ -233,7 +233,9 @@ bool Source::handlePartialTile(const TileID& id, Worker&) {
         return true;
     }
 
-    util::ptr<TileData> data = it->second.lock();
+    // Note: this uses a raw pointer; we don't want the callback binding to have a
+    // shared pointer.
+    VectorTileData* data = static_cast<VectorTileData*>(it->second.lock().get());
     if (!data) {
         return true;
     }
@@ -241,14 +243,12 @@ bool Source::handlePartialTile(const TileID& id, Worker&) {
     // The signal is only emitted if there was an actual change on the tile. The
     // tile can be in a "partial" state waiting for resources and get reparsed on
     // the arrival of new resources that were needed by another tile.
-    size_t bucketCount = static_cast<VectorTileData*>(data.get())->countBuckets();
-    auto callback = [this, data, bucketCount]() {
-        if (static_cast<VectorTileData*>(data.get())->countBuckets() > bucketCount) {
+    size_t bucketCount = data->countBuckets();
+    return data->reparse([this, data, bucketCount]() {
+        if (data->countBuckets() > bucketCount) {
             emitTileLoaded(false);
         }
-    };
-
-    return data->reparse(callback);
+    });
 }
 
 TileData::State Source::addTile(MapData& data,