screencast: Properly stop all recordings on session mode changes

We currently just clear out the map of recorders (which results
in the top bar indicator to be hidden), but don't stop the actual
recordings.
Spotted by Adel Gadllah on IRC.

js/ui/screencast.js

@@ -68,8 +68,8 @@ const ScreencastService = new Lang.Class({
         if (Main.sessionMode.allowScreencast)
             return;
 
-        this._recorders.clear();
-        this.emit('updated');
+        for (let sender of this._recorders.keys())
+            this._stopRecordingForSender(sender);
     },
 
     _onNameVanished: function(connection, name) {