patch 8.1.2071: when 'wincolor' is set text property changes highlighting

Problem:    When 'wincolor' is set text property changes highlighting. (Andy
            Stewart)
Solution:   Fix combining colors. (closes #4968)
diff --git a/src/testdir/test_highlight.vim b/src/testdir/test_highlight.vim
index 27d7641..7db33bf 100644
--- a/src/testdir/test_highlight.vim
+++ b/src/testdir/test_highlight.vim
@@ -595,8 +595,13 @@
 
   let lines =<< trim END
 	set cursorline cursorcolumn rnu
-	call setline(1, ["","1111111111","22222222222","3 here 3",""])
+	call setline(1, ["","1111111111","22222222222","3 here 3","","the cat is out of the bag"])
 	set wincolor=Pmenu
+	hi CatLine guifg=green ctermfg=green
+	hi Reverse gui=reverse cterm=reverse
+	syn match CatLine /^the.*/
+	call prop_type_add("foo", {"highlight": "Reverse", "combine": 1})
+	call prop_add(6, 12, {"type": "foo", "end_col": 15})
 	/here
   END
   call writefile(lines, 'Xtest_wincolor')