UI: Remove all unused user sources on load

This temporarily fixes an issue where potential (for whatever reason)
stray sources that aren't associated with any scenes could be saved and
then persist with the save data on load.

obs/window-basic-main.cpp

@@ -415,6 +415,30 @@ void OBSBasic::LoadSceneListOrder(obs_data_array_t *array)
 	}
 }
 
+void OBSBasic::CleanupUnusedSources()
+{
+	auto removeUnusedSources = [&](obs_source_t *source)
+	{
+		obs_scene_t *scene = obs_scene_from_source(source);
+		if (scene)
+			return;
+
+		if (sourceSceneRefs[source] == 0) {
+			sourceSceneRefs.erase(source);
+			obs_source_remove(source);
+		}
+	};
+	using func_type = decltype(removeUnusedSources);
+
+	obs_enum_sources(
+			[](void *f, obs_source_t *source)
+			{
+				(*static_cast<func_type*>(f))(source);
+				return true;
+			},
+			static_cast<void*>(&removeUnusedSources));
+}
+
 void OBSBasic::Load(const char *file)
 {
 	if (!file) {
@@ -478,6 +502,8 @@ void OBSBasic::Load(const char *file)
 
 	obs_data_release(data);
 
+	CleanupUnusedSources();
+
 	disableSaving--;
 }