patch 8.2.1514: multibyte vertical separator is cleared when dragging popup

Problem:    Multibyte vertical separator is cleared when dragging a popup
            window using a multi-byte character for the border.
Solution:   Only clear the character before the window if it is double width.
            (closes #6766)

src/screen.c

@@ -465,7 +465,8 @@ screen_line(
     // double-wide character. Clear the left half to avoid it getting the popup
     // window background color.
     if (coloff > 0 && ScreenLines[off_to] == 0
-					     && ScreenLinesUC[off_to - 1] != 0)
+		   && ScreenLinesUC[off_to - 1] != 0
+		   && (*mb_char2cells)(ScreenLinesUC[off_to - 1]) > 1)
     {
 	ScreenLines[off_to - 1] = ' ';
 	ScreenLinesUC[off_to - 1] = 0;

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1514,
 /**/
     1513,
 /**/