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/dumps/Test_wincolor_01.dump b/src/testdir/dumps/Test_wincolor_01.dump
index 06f45b8..5a5ccf5 100644
--- a/src/testdir/dumps/Test_wincolor_01.dump
+++ b/src/testdir/dumps/Test_wincolor_01.dump
@@ -1,8 +1,8 @@
 | +0#af5f00255#ffd7ff255@1|2| | +0#0000001&@4| +0&#e0e0e08| +0&#ffd7ff255@64
-| +0#af5f00255&@1|1| |1+0#0000001&@4|1+0#0000000#e0e0e08@4| | +0#0000001#ffd7ff255@59
-| +8#af5f00255&@1|0| |2+0#0000000#e0e0e08@4>2+0#0000001#ffd7ff255@5| +8&&@59
+| +0#af5f00255&@1|1| |1+0#0000001&@4|1+0&#e0e0e08@4| | +0&#ffd7ff255@59
+| +8#af5f00255&@1|0| |2+0#0000001#e0e0e08@4>2+0&#ffd7ff255@5| +8&&@59
 | +0#af5f00255&@1|1| |3+0#0000001&| |h|e|r|e+0&#e0e0e08| +0&#ffd7ff255|3| @62
 | +0#af5f00255&@1|2| | +0#0000001&@4| +0&#e0e0e08| +0&#ffd7ff255@64
+| +0#af5f00255&@1|3| |t+0#40ff4011&|h|e| |c|a+0&#e0e0e08|t+0&#ffd7ff255| |i|s| |o+1&&|u|t| +0&&|o|f| |t|h|e| |b|a|g| +0#0000001&@45
 |~+0#4040ff13&| @73
-|~| @73
 |-+2#0000000#ffffff0@1| |V|I|S|U|A|L| |-@1| +0&&@34|2| @8|3|,|6| @10|A|l@1| 
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')