xwayland: reset cursor image on cursor theme reload

xwayland: reset cursor image on cursor theme reload

As wlr_xwayland caches the pixel data when not yet started
up due to the delayed lazy startup approach, we do have to
re-set the xwayland cursor image when reloading the cursor
theme. Otherwise the first X11 client connected will cause
the xwayland server to use the cached (and destroyed) pixel
data.

To reproduce:
- Compile with b_sanitize=address,undefined
- Start labwc (nothing in autostart that could create
  a X11 connection, e.g. no GTK or X11 application)
- Reconfigure
- Start some X11 client