patch 8.0.1097: background color wrong if job changes background color

Problem:    Background color wrong if job changes background color.
Solution:   Get the background color from vterm.
diff --git a/src/screen.c b/src/screen.c
index 40c9a15..42995c7 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -3257,7 +3257,7 @@
     {
 	extra_check = TRUE;
 	get_term_attr = TRUE;
-	term_attr = term_get_attr(wp->w_buffer, 0, 0);
+	term_attr = term_get_attr(wp->w_buffer, lnum, -1);
     }
 #endif