Track each panel separately via its wl_resource. `set_panel_position`
might be called before `set_panel`, so reuse panel config.
Place the position in panel_config so that each panel has its own
position.
This does not work as expected. I think the problem is on the wlc side.
Please review, @Cloudef. To reproduce the issues:
1. Run sway
2. Open terminal in sway
3. Run swaybg
swaybg will create a surface and ask to have it set as the background,
but wlc_handle_from_wl_surface_resource will return 0. If the swaybg
surface is a shell surface, then it works - but wlc complains about the
pointer type and segfaults as soon as the pre-render hook tries to draw
the background.
