screenshot: Move HAVE_RECORDER check into screencast service

Some gstreamer plugins require a connection to the display server,
so if we end up initializing gstreamer before we are ourselves
fully initialized, we may end up with a locked compositor.

Avoid this by moving the runtime recorder check into the screencast
D-Bus service, so that all gstreamer calls happen out of process.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5710

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2519>

js/dbusServices/screencast/screencastService.js

@@ -258,9 +258,17 @@ var Recorder = class {
 };
 
 var ScreencastService = class extends ServiceImplementation {
+    static canScreencast() {
+        return Gst.init_check(null) &&
+            Gst.ElementFactory.find('pipewiresrc') &&
+            Gst.ElementFactory.find('filesink');
+    }
+
     constructor() {
         super(ScreencastIface, '/org/gnome/Shell/Screencast');
 
+        this._canScreencast = ScreencastService.canScreencast();
+
         Gst.init(null);
         Gtk.init();
 
@@ -280,6 +288,10 @@ var ScreencastService = class extends ServiceImplementation {
             '/org/gnome/Shell/Introspect');
     }
 
+    get ScreencastSupported() {
+        return this._canScreencast;
+    }
+
     _removeRecorder(sender) {
         this._recorders.delete(sender);
         if (this._recorders.size === 0)