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: #17081

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/options.txt

@@ -3621,6 +3621,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  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.
 
@@ -3645,6 +3646,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  lastline	NonText			|hl-NonText|
 	  trunc		one of the many Popup menu highlighting groups like
 			|hl-PmenuSel|
+	  truncrl	same as "trunc"
 
 						*'findfunc'* *'ffu'* *E1514*
 'findfunc' 'ffu'	string	(default empty)

runtime/doc/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2025 Apr 08
+*version9.txt*  For Vim version 9.1.  Last change: 2025 Apr 12
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -41631,6 +41631,7 @@ Options: ~
   and CTRL-D / CTRL-U for half-pagewise scrolling
 - New option value for 'fillchars':
   	"trunc"		- configure truncation indicator, 'pummaxwidth'
+  	"truncrl"	- like "trunc" but in 'rl' mode, 'pummaxwidth'
 
 Ex commands: ~
 - allow to specify a priority when defining a new sign |:sign-define|