fix problem where displaying multiline regexes that have their start on

fix problem where displaying multiline regexes that have their start on
one page of a line and their end on a different page of the same line
would paint past the end of the regex by tweaking DB's changes to add
multibyte string support to edit_add(); now calculate paintlen the same
way the original working code did, but keep accounting for multibyte
characters as DB'S new code did

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2253 35c25a1d-7b9e-4130-9fde-d3aeb78583b8