Invisibility spec of this buffer.
The default is t, which means that text is invisible
if it has a non-nil `invisible' property.
If the value is a list, a text character is invisible if its `invisible'
property is an element in that list (or is a list with members in common).
If an element is a cons cell of the form (PROP . ELLIPSIS),
then characters with property value PROP are invisible,
and they have an ellipsis as well if ELLIPSIS is non-nil.