Set panels' geometries correctly and don't render them explicitly

Panels were explicitly rendered by calling wlc_surface_render in
handle_output_pre_render. Calling wlc_surface_render does not set the
surface's geometry (like wlc_view_set_geometry does). Sway did not call
wlc_view_set_geometry for panels, so wlc defaulted their geometry to be at
the origin. This is not correct for bars unless their location is top.

Furthermore, for a surface to receive pointer events, its mask has to be
set to visible. This causes wlc to render these surfaces, causing panels
and backgrounds to be rendered twice.

This commit makes panels and surfaces visible, sets the correct geometries
and removes the code that explicitly rendered them.

include/extensions.h

@@ -23,6 +23,8 @@ struct panel_config {
 	enum desktop_shell_panel_position panel_position;
 	// used to determine if client is a panel
 	struct wl_client *client;
+	// wlc handle for this panel's surface, not set until panel is created
+	wlc_handle handle;
 };
 
 struct desktop_shell_state {