screenshot: Enable opacity redirect for the panel

This makes the panel look correct when it fades out (e.g. while dragging
the area selection around or while the screenshot UI is fading in and
out).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2138>

js/ui/screenshot.js

@@ -1081,6 +1081,7 @@ var ScreenshotUI = GObject.registerClass({
             y_align: Clutter.ActorAlign.END,
             y_expand: true,
             vertical: true,
+            offscreen_redirect: Clutter.OffscreenRedirect.AUTOMATIC_FOR_OPACITY,
         });
         this._primaryMonitorBin.add_child(this._panel);