Before this change, the `GeoJSONVTData` instance was retained at the scheduled
lambda, which run on the worker thread represented by the `GeoJSONVTData::scheduler`
class member:
```
std::weak_ptr<GeoJSONVTData> weak = shared_from_this();
scheduler->scheduleAndReplyValue(
[id, weak, this]() -> TileFeatures {
if (auto self = weak.lock()) {
return impl.getTile(id.z, id.x, id.y).features;
}
return {};
},
fn);
```
It caused program termination in case `self` turned to be the last reference to `this`,
as the `std::thread` destructor was called from the thread it represented.
Now, only the `GeoJSONVTData::impl` class member is retained.
3ea459233a