cursor: Try to fall back to a default xcursor theme first

The use case is systems where for some reason the current xcursor theme
cannot be accessed (an application packaged as a strictly confined snap,
for example).

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.

Fixes: https://gitlab.gnome.org/Community/Ubuntu/gnome-sdk/-/issues/6



Signed-off-by: Olivier Tilloy <olivier.tilloy@canonical.com>