patch 8.1.1071: cannot get composing characters from the screen

Problem:    Cannot get composing characters from the screen.
Solution:   Add screenchars() and screenstring(). (partly by Ozaki Kiichi,
            closes #4059)
diff --git a/src/testdir/view_util.vim b/src/testdir/view_util.vim
index 29ea073..34b4519 100644
--- a/src/testdir/view_util.vim
+++ b/src/testdir/view_util.vim
@@ -5,6 +5,7 @@
   finish
 endif
 
+" Get text on the screen, without composing characters.
 " ScreenLines(lnum, width) or
 " ScreenLines([start, end], width)
 function! ScreenLines(lnum, width) abort
@@ -23,6 +24,25 @@
   return lines
 endfunction
 
+" Get text on the screen, including composing characters.
+" ScreenLines(lnum, width) or
+" ScreenLines([start, end], width)
+function! ScreenLinesUtf8(lnum, width) abort
+  redraw!
+  if type(a:lnum) == v:t_list
+    let start = a:lnum[0]
+    let end = a:lnum[1]
+  else
+    let start = a:lnum
+    let end = a:lnum
+  endif
+  let lines = []
+  for l in range(start, end)
+    let lines += [join(map(range(1, a:width), 'screenstring(l, v:val)'), '')]
+  endfor
+  return lines
+endfunction
+
 function! ScreenAttrs(lnum, width) abort
   redraw!
   if type(a:lnum) == v:t_list