bug#39360: Add :extend attribute to show-paren-mode expression face

[Eli Zaretskii]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: Eli Zaretskii <eliz@gnu.org>,  39360@debbugs.gnu.org
> Date: Fri, 31 Jan 2020 17:03:41 +0100
> 
> What also looks weird and infelicitous to me is that a nonextending
> background color face on multiple lines shows up on the empty space at
> the beginning of a line, i.e., it appears to extend into this space, see
> attached screenshot.  I think this only happens with white space
> characters, not with lines indented e.g. with wrap-prefix.  It would be
> nice to treat all "empty" space the same in this respect.

That space is not empty, there are characters there.