glwindow: Add "window-handle-changed" signal

This allow other objects to clear any wrapper object that depends
on the previous handle, and properly re-create the new wrappers without
having to resort into doing pointer comparison.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1231>

gst-libs/gst/gl/gstglwindow.c

@@ -127,6 +127,7 @@ enum
   EVENT_MOUSE_SIGNAL,
   EVENT_KEY_SIGNAL,
   EVENT_SCROLL_SIGNAL,
+  WINDOW_HANDLE_CHANGED_SIGNAL,
   LAST_SIGNAL
 };
 
@@ -253,6 +254,22 @@ gst_gl_window_class_init (GstGLWindowClass * klass)
       G_TYPE_NONE, 4, G_TYPE_DOUBLE, G_TYPE_DOUBLE, G_TYPE_DOUBLE,
       G_TYPE_DOUBLE);
 
+  /**
+   * GstGLWindow::window-handle-changed:
+   * @object: the #GstGLWindow
+   *
+   * Will be emitted when the window handle has been set into the native
+   * implementation, but before the context is re-activated. By using this
+   * signal, elements can refresh associated resource without relying on
+   * direct handle comparision.
+   *
+   * Since: 1.20
+   */
+  gst_gl_window_signals[WINDOW_HANDLE_CHANGED_SIGNAL] =
+      g_signal_new ("window-handle_changed", G_TYPE_FROM_CLASS (klass),
+      G_SIGNAL_RUN_LAST, 0, NULL, NULL, g_cclosure_marshal_generic,
+      G_TYPE_NONE, 0);
+
   _init_debug ();
 }
 
@@ -381,6 +398,12 @@ _set_window_handle_cb (GstSetWindowHandleCb * data)
 
   window_class->set_window_handle (data->window, data->handle);
 
+  /* Let's assume users don't call this without a new handle, this is the
+   * safest since we don't know the nature of the handle and direct
+   * comparision might not be safe */
+  g_signal_emit (data->window,
+      gst_gl_window_signals[WINDOW_HANDLE_CHANGED_SIGNAL], 0, NULL);
+
   /* reactivate */
   if (context && thread)
     gst_gl_context_activate (context, TRUE);