osd: prevent preview outlines from overlapping OSD in first output

osd: prevent preview outlines from overlapping OSD in first output

Following code didn't consider multi-monitor setup:

  wlr_scene_node_raise_to_top(&server->menu_tree->node);
  wlr_scene_node_raise_to_top(&output->osd_tree->node);

...because when executed twice, server->menu_tree is raised above
output->osd_tree for the first output. This resulted in a scene tree like
(higher to lower):

- output->osd_tree (for 2nd output)
- output->server->osd_state.preview_outline (placed just above menu_tree)
- sever->menu_tree
- output->osd_tree (for 1st output)

So the previous outline for window switcher is placed above the OSD in the
first output.