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/custom_geometry_source.cpp

@@ -1,6 +1,7 @@
 #include <mbgl/style/sources/custom_geometry_source.hpp>
 #include <mbgl/style/custom_tile_loader.hpp>
 #include <mbgl/style/sources/custom_geometry_source_impl.hpp>
+#include <mbgl/style/source_observer.hpp>
 #include <mbgl/actor/actor.hpp>
 #include <mbgl/actor/scheduler.hpp>
 #include <mbgl/tile/tile_id.hpp>
@@ -25,6 +26,7 @@ const CustomGeometrySource::Impl& CustomGeometrySource::impl() const {
 void CustomGeometrySource::loadDescription(FileSource&) {
     baseImpl = makeMutable<CustomGeometrySource::Impl>(impl(), loader->self());
     loaded = true;
+    observer->onSourceLoaded(*this);
 }
 
 void CustomGeometrySource::setTileData(const CanonicalTileID& tileID,

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;
     }
 

src/mbgl/style/sources/vector_source.cpp

@@ -38,6 +38,7 @@ void VectorSource::loadDescription(FileSource& fileSource) {
     if (urlOrTileset.is<Tileset>()) {
         baseImpl = makeMutable<Impl>(impl(), urlOrTileset.get<Tileset>());
         loaded = true;
+        observer->onSourceLoaded(*this);
         return;
     }
 

src/mbgl/style/style_impl.cpp

@@ -140,9 +140,8 @@ void Style::Impl::addSource(std::unique_ptr<Source> source) {
     }
 
     source->setObserver(this);
-    source->loadDescription(fileSource);
-
-    sources.add(std::move(source));
+    auto item = sources.add(std::move(source));
+    item->loadDescription(fileSource);
 }
 
 std::unique_ptr<Source> Style::Impl::removeSource(const std::string& id) {