wayland: Reset tablet tool cursor surface on proximity out

The cursor surface would be remembered until the next proximity in
event, causing flashing of the old cursor till the client underneath
the tablet tool sent the zwp_tablet_tool.set_cursor request.

Forgetting about the cursor surface on proximity out makes the cursor
invisible till the request is made.

src/wayland/meta-wayland-tablet-tool.c

@@ -914,6 +914,7 @@ meta_wayland_tablet_tool_update (MetaWaylandTabletTool *tool,
       break;
     case CLUTTER_PROXIMITY_OUT:
       tool->current_tablet = NULL;
+      meta_wayland_tablet_tool_set_cursor_surface (tool, NULL);
       meta_wayland_tablet_tool_update_cursor_surface (tool);
       g_clear_object (&tool->cursor_renderer);
       break;