Make the FileSource available from the ThreadContext

For now we set the FileSource for the Worker context
but in the future after fixing #1664 we can assert()
that FileSource are set only for the Map thread context.

test/style/resource_loading.cpp

@@ -29,6 +29,7 @@ public:
           envScope_(env_, ThreadType::Map, "Map"),
           data_(view, MapMode::Still),
           callback_(callback) {
+        util::ThreadContext::setFileSource(&fileSource);
 
         data_.transform.resize(1000, 1000, 1.0, 1000, 1000);
         data_.transform.setLatLngZoom({0, 0}, 16);