swipeTracker: Ignore Meta key while workspace scroll gesture is in progress

Workspace transition stopped midway when the Meta key
was released while the two-finger scroll gesture was
in progress. This commit ignores the Meta key once
the gesture has been confirmed and is in motion.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2135>

js/ui/swipeTracker.js

@@ -364,7 +364,7 @@ const ScrollGesture = GObject.registerClass({
         if ((this._allowedModes & Main.actionMode) === 0)
             return false;
 
-        if (this.scrollModifiers !== 0 &&
+        if (!this._began && this.scrollModifiers !== 0 &&
             (event.get_state() & this.scrollModifiers) === 0)
             return false;