bug#17234: 24.3.50; overlay priority : cons cells make an error in ediff

[Stefan Monnier]

> Then how can we expect authors of other Lisp packages to be able to
> fix their code so it works with Emacs 24.4 and later?  We must say
> _something_ in NEWS.

It has always been the case that `priority' could be any value.

Any non-number value was treated as nil by the display engine, whereas
now some cons values are treated as something else.

But ediff's bug could already be triggered in Emacs<24.4 by a package
installing an overlay with a `priority' that's a cons cell.


        Stefan