try to decode the image header as soon as some data is available

src/asset.h

@@ -17,6 +17,7 @@ public:
     bool m_stop_async = false;
     std::function<bool(float)> on_progress;
     std::future<bool> remote_future;
+    std::vector<uint8_t> tmp_data;
 
     std::string m_current_path;
     std::string m_current_url;