Introduce `-[MGLSource initWithPendingSource:]`, allowing the base class to track the owned `_pendingSource` pointer and implement `-addToMapView:` and `-removeFromMapView:` without any casts.
Introduce `-[MGLSource initWithPendingSource:]`, allowing the base class to track the owned `_pendingSource` pointer and implement `-addToMapView:` and `-removeFromMapView:` without any casts.