Fl_Text_Buffer/Display fixes for UTF-8 / STR-2158 (part 1)

fixes to handle incorrect counting of UTF-8 characters by checking
for complete UTF-8 encodings in char* rather than char by char.



git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@7527 ea41ed52-d2ee-0310-a9c1-e6b18d33e121