dnd: Only handle touch events in wayland

There are serveral issues around touch passive grab and touch/pointer doubly
handling to use these on X11, so we stick to single-touch/pointer there.

Cherry picked from commit 60ccdc2deb

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1015

js/ui/dnd.js

@@ -125,6 +125,16 @@ var _Draggable = new Lang.Class({
     },
 
     _onTouchEvent(actor, event) {
+        // Here we only handle touch events on wayland. On X11
+        // we do get emulated pointer events, which already works
+        // for single-touch cases. Besides, the X11 passive touch grab
+        // set up by Mutter will make us see first the touch events
+        // and later the pointer events, so it will look like two
+        // unrelated series of events, we want to avoid double handling
+        // in these cases.
+        if (!Meta.is_wayland_compositor())
+            return Clutter.EVENT_PROPAGATE;
+
         if (event.type() != Clutter.EventType.TOUCH_BEGIN ||
             !global.display.is_pointer_emulating_sequence(event.get_event_sequence()))
             return Clutter.EVENT_PROPAGATE;