Draw: Reduce overhead of sharing meshes without attributes

When using a lot of instances the requested and needed attributes
are merged. This process uses a lock even when no work needs to be
done.

By early exiting the merging process when no work needs to be done
the performance of navigating 60k cubes went from 17.5 fps to 18.3 fps.

Detected when researching #126391.

Pull Request: https://projects.blender.org/blender/blender/pulls/129791

source/blender/draw/intern/draw_attributes.cc

@@ -50,6 +50,9 @@ void drw_attributes_clear(DRW_Attributes *attributes)
 
 void drw_attributes_merge(DRW_Attributes *dst, const DRW_Attributes *src, std::mutex &render_mutex)
 {
+  if (src->num_requests == 0) {
+    return;
+  }
   std::lock_guard lock{render_mutex};
   drw_attributes_merge_requests(src, dst);
 }