screenshot: Make screencast area indicator hidden from pick

It's a non-reactive actor on top of most of the stage. Set it as hidden
from pick so it doesn't break in-Shell DnD.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5083

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2192>

js/ui/screenshot.js

@@ -1047,6 +1047,7 @@ var ScreenshotUI = GObject.registerClass({
             GObject.BindingFlags.DEFAULT);
         // Add it directly to the stage so that it's above popup menus.
         global.stage.add_child(this._screencastAreaIndicator);
+        Shell.util_set_hidden_from_pick(this._screencastAreaIndicator, true);
 
         Main.layoutManager.screenshotUIGroup.add_child(this);