For tablet tools if we have NULL cursor, we use the default cursor
instead. This provides us with a tablet cursor when an application never
sets the cursor.

However, on proximity out when we clear said cursor we also
need to toggle off cursor_is_default, otherwise on the next proximity in
we assume we already have a cursor and never update it again.

This leads to an invisible cursor over GTK application when the tablet
tool is brought into proximity over the widget (but not when moving into
the widget from the outside).

Closes: #6312
(cherry picked from commit 04870fc1)