Cannot add MGL*StyleLayer for custom vector sources

[boundsj]

Platform: iOS
Mapbox SDK version: 3.4.0 alpha 4

Steps to trigger behavior

1. Create and locally host a TileJSON document that points to a valid vector tileset (i.e. [https://vector.mapzen.com/osm/all/{z}/{x}/{y}.mvt]
2. Create an MGLVectorSource that uses a URL pointed at the previously created TileJSON (adjust ATS permissions to allow this on iOS)
3. Create a style layer (i.e. MGLFillStyleLayer) for the vector source and add it to the map's style

Expected behavior

You see the vector source data visually represented by the added style.

Actual behavior

No change to the map.

cc @1ec5 @frederoni

[boundsj]

With the fix for #6252 came the ability to address and style vector source layers. Since 3.4.0 alpha 4 you can style the layers of a vector source that you host. For example, assuming you are hosting a TileJSON document locally that points to Mapzen's vector tile service, you can write:

- (void)mapViewDidFinishLoadingMap:(MGLMapView *)mapView {
    NSURL *url = [NSURL URLWithString:@"http://localhost:8000/osm.layer"];
    MGLVectorSource *vectorSource = [[MGLVectorSource alloc] initWithSourceIdentifier:@"mapzen" URL:url];
    [self.mapView.style addSource:vectorSource];

    MGLFillStyleLayer *fillLayer = [[MGLFillStyleLayer alloc] initWithLayerIdentifier:@"filllayer" source:vectorSource sourceLayer:@"buildings"];
    fillLayer.fillColor = [UIColor magentaColor];
    [self.mapView.style addLayer:fillLayer];
}

and see the filled buildings:

[boundsj]

Correction: the ability to address a sources layer does exist on master but not in a published alpha release. The next alpha release will have this functionality.