wayland: Unset button modifiers on pointer enter

Due to implicit grabs, we basically can guarantee that the pointer
won't have any buttons pressed at the time of wl_pointer.enter.
Seems like a good place to unset any button modifiers that might
have been left stale by compositor grabs.

gdk/wayland/gdkdevice-wayland.c

@@ -1072,6 +1072,8 @@ pointer_handle_enter (void              *data,
   device->pointer_focus = wl_surface_get_user_data(surface);
   g_object_ref(device->pointer_focus);
 
+  device->button_modifiers = 0;
+
   device->surface_x = wl_fixed_to_double (sx);
   device->surface_y = wl_fixed_to_double (sy);
   device->enter_serial = serial;