bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[martin rudalics]

> Please push it for me.

Pushed now.  Please have a look.

One more question: Are we sure that we want to show the background
(underline, ...) on the newline character when we do not extend?  It
certainly provides the information that the character is covered by
the property or overlay at that position and also fits well when the
cursor is positioned on it.  Yet it appears somewhat clumsy.

martin