title: Is flutter's `fl_texture_registrar_mark_texture_frame_available` thread safe? Can it be called from other threads?
body: <p>Whenever you update a flutter external texture in linux, you have to mark it as available so it knows that it has to re-render it. This is done with <a href="https://github.com/flutter/engine/blob/2962099077b36704071802ea5595f4a016a1c214/shell/platform/linux/public/flutter_linux/fl_texture_registrar.h#L73" rel="nofollow noreferrer">fl_texture_registrar_mark_texture_frame_available</a>, like this:</p>
  <pre><code>fl_texture_registrar_mark_texture_frame_available(texture_registrar,
                                                          texture);
  </code></pre>
  <p>In which thread should <code>fl_texture_registrar_mark_texture_frame_available</code> be called? Usually, texture stuff is managed into a plugin, so we don't even have access to Flutter's main thread (I guess). Can I call this from other threads? Is it thread safe?</p>