cursor-shape: use server-side cursors for custom (OSC-22), if possible

Using a lookup table, try to map the user-provided xcursor string to a
cursor-shape-v1 known shape.

If we succeed, set the user’s custom cursor using server side
cursors (i.e. using cursor-shape-v1).

If not, fallback to trying to load the image ourselves (using
wl_cursor_theme_get_cursor()), and set it using the legacy
wl_pointer_set_cursor().

wayland.h

@@ -159,6 +159,8 @@ struct seat {
         float scale;
         bool hidden;
         enum cursor_shape shape;
+        char *last_custom_xcursor;
+
         struct wl_callback *xcursor_callback;
         bool xcursor_pending;
     } pointer;