Improves FLTK usage on macintosh computers with retina displays.
The editor demo had grey lines between text lines and its cursor left traces behind
when moved. The scroll demo left some faint lines behind also.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10401 ea41ed52-d2ee-0310-a9c1-e6b18d33e121