cea608overlay: Fix black-background setting

Apply the property to newly created renderer

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1542>

video/closedcaption/src/cea608overlay/imp.rs

@@ -529,6 +529,9 @@ impl ElementImpl for Cea608Overlay {
                     -1 => None,
                     val => Some(val as u8),
                 };
+                state
+                    .renderer
+                    .set_black_background(settings.black_background);
             }
             _ => (),
         }