Try to fall back to a default xcursor theme if the requested theme isn't available.

The use case is systems where for some reason the current xcursor theme cannot be accessed. Before falling back to wayland's default cursor theme, it is worth trying the xcursor theme called "default", which hopefully looks better than the former.

Snap applications running on Wayland can access a well-known subset of icon and cursor themes (through the gtk-common-themes content snap), but if the host system is configured to use a custom cursor theme that isn't in that subset, this results in rather ugly cursors for those snapped applications. The content snap that provides well-known themes symlinks the theme called "default" to DMZ-White. See this bug for details.