This is the first concrete step to remove promises from the parsing
pipeline. Buckets depending on the Sprite (i.e. SymbolBucket) won't
be created if the Sprite is not loaded yet. When the Sprite finally gets
loaded, it will fire a signal that will ultimately trigger a reparse.
We still have to expose the Sprite to the Map object as it is needed
for getting offset pixels for annotation symbols, but we can probably
improve that.
ResourceLoader will handle all the tile changes and notifications
through the Source object owned by the Style. This object lives
entirely on the Map thread and should be only accessed by it.
The observer will receive notifications every time the tile data changes
and this will probably trigger a new rendering operation.
This first patch will delegate Source loading to the ResourceLoader.
