Set default cursor image if Wayland client doesn't

If a Wayland client doesn't set a cursor image, we currently fall back to the last cursor image some other client set. This often results in us showing a resize or text input image, looking weird.

Weston appears to set the default cursor in those cases - lets do the same.

This is easily reproducible with simple client such as weston-simple-damage and friends, but came to my attention in Georges streams when opening a nested compositor.

Note: this might affect unresponsive clients as well, haven't checked yet. It might be desirable in those cases as well, either way.

Update: Weston does not do this (any more)? But KDE now does appear to do so somehow.