bug#20802: Segfault when showing non-GTK+ tooltip

[martin rudalics]

> Why can't we simply move the code that frees the image cache to
> delete_terminal?  There's only one image cache for each terminal, and
> it's shared by all frames on that terminal, right?  And we call
> delete-terminal when we delete the last frame on the terminal, right?

‘delete-terminal’ can run into precisely the same problem I described in
my previous scenario.  IMHO we have to give up the idea of using
refcounts to tell whether a frame might still use some object - they are
not up to this task.

martin