patch 8.2.1509: vertical separator is cleared when dragging a popup window

Problem:    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 using a
            multi-byte character. (closes #6766)

Author: brammool

src/screen.c

@@ -464,7 +464,8 @@ screen_line(
     // First char of a popup window may go on top of the right half of a
     // double-wide character. Clear the left half to avoid it getting the popup
     // window background color.
-    if (coloff > 0 && ScreenLines[off_to] == 0)
+    if (coloff > 0 && ScreenLines[off_to] == 0
+					     && ScreenLinesUC[off_to - 1] != 0)
     {
 	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 */
+/**/
+    1509,
 /**/
     1508,
 /**/