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

[martin rudalics]

> 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.

It would be nice to have that.  But I doubt that it can be done.

martin