wayland: Add getter for the current surface of a tablet device

That would be the surface under the tool that is being currently used
on the device. This will be used by MetaWaylandSeat to implement the
default MetaWaylandEventInterface.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3420>

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

@@ -937,3 +937,16 @@ meta_wayland_tablet_tool_can_popup (MetaWaylandTabletTool *tool,
 {
   return tool->down_serial == serial || tool->button_serial == serial;
 }
+
+gboolean
+meta_wayland_tablet_tool_has_current_tablet (MetaWaylandTabletTool *tool,
+                                             MetaWaylandTablet     *tablet)
+{
+  return tool->current_tablet == tablet;
+}
+
+MetaWaylandSurface *
+meta_wayland_tablet_tool_get_current_surface (MetaWaylandTabletTool *tool)
+{
+  return tool->current;
+}