asyncresourcegatherer: clarify callback thread safety

src/renderer/AsyncResourceGatherer.hpp

@@ -38,7 +38,9 @@ class CAsyncResourceGatherer {
 
         std::unordered_map<std::string, std::any> props;
 
-        // optional
+        // optional. Callbacks will be dispatched from the main thread,
+        // so wayland/gl calls are OK.
+        // will fire once the resource is fully loaded and ready.
         void (*callback)(void*) = nullptr;
         void* callbackData      = nullptr;
     };