vim-patch:9.1.1296: completion: incorrect truncation logic

Problem:  completion: incorrect truncation logic (after: v9.1.1284)
Solution: replace string allocation with direct screen rendering and
          fixe RTL/LTR truncation calculations (glepnir)

closes: vim/vim#17081

d4dbf822dc

Co-authored-by: glepnir <glephunter@gmail.com>

src/nvim/buffer_defs.h

@@ -1057,6 +1057,7 @@ typedef struct {
   schar_T eob;
   schar_T lastline;
   schar_T trunc;
+  schar_T truncrl;
 } fcs_chars_T;
 
 /// Structure which contains all information that belongs to a window.