cursor-tracker: Add way to force tracking cursor position

On X11 we won't always receive cursor positions, as some other client
might have grabbed the pointer (e.g. for implementing a popup menu). To
make screen casting show a somewhat correct cursor position, we need to
actively poll the X server about the current cursor position.

We only really want to do this when screen casting or taking a
screenshot, so add an API that forces the cursor tracker to track the
cursor position.

On the native backend this is a no-op as we by default always track the
cursor position anyway.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1391

src/backends/meta-cursor-renderer.c

@@ -383,8 +383,6 @@ meta_cursor_renderer_set_position (MetaCursorRenderer *renderer,
 {
   MetaCursorRendererPrivate *priv = meta_cursor_renderer_get_instance_private (renderer);
 
-  g_assert (meta_is_wayland_compositor ());
-
   priv->current_x = x;
   priv->current_y = y;