svg_loader: allow deferred loading.

don't push thread to work hard unless it's necessary.
that might occur performance drop.

Change-Id: Ib71fa0ae46461b10b7ed7ef356512fbf2a7aa0a8

src/loaders/svg_loader/tvgSvgLoader.cpp

@@ -2295,7 +2295,7 @@ bool SvgLoader::read()
         return loader->builder.build(loader->loaderData.doc);
     };
 
-    rootProgress = async(launch::async, asyncTask, this);
+    rootProgress = async((launch::async | launch::deferred), asyncTask, this);
 
     return true;
 }