Always call onSourceLoaded observers (#15548)

* [core] add sources to source collection before triggering load

* [test] add testcase for #15514

* [core] also call onSourceLoaded observers when no network request was necessary

src/mbgl/style/sources/raster_source.cpp

@@ -41,6 +41,7 @@ void RasterSource::loadDescription(FileSource& fileSource) {
     if (urlOrTileset.is<Tileset>()) {
         baseImpl = makeMutable<Impl>(impl(), urlOrTileset.get<Tileset>());
         loaded = true;
+        observer->onSourceLoaded(*this);
         return;
     }