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/options.lua

@@ -3062,6 +3062,7 @@ local options = {
           lastline	'@'		'display' contains lastline/truncate
           trunc		'>'		truncated text in the
         				|ins-completion-menu|.
+          truncrl	'<'		same as "trunc' in 'rightleft' mode
 
         Any one that is omitted will fall back to the default.
 
@@ -3101,6 +3102,7 @@ local options = {
           lastline	NonText			|hl-NonText|
           trunc		one of the many Popup menu highlighting groups like
         		|hl-PmenuSel|
+          truncrl	same as "trunc"
       ]=],
       expand_cb = 'expand_set_chars_option',
       full_name = 'fillchars',