rootston: focus newly-created surfaces

Whenever a new surface is created, we have to update the cursor focus,
even if there's no input event. So, we generate one motion event, and
reuse the code to update the proper cursor focus. We need to do this
for all surface roles - toplevels, popups, subsurfaces.

Fixes #1162

include/rootston/input.h

@@ -32,4 +32,6 @@ struct roots_seat *input_get_seat(struct roots_input *input, char *name);
 
 struct roots_seat *input_last_active_seat(struct roots_input *input);
 
+void input_update_cursor_focus(struct roots_input *input);
+
 #endif