When we have more thumbnails than what fits on the primary display's width and
press Alt+Shift+Above_Tab the thumbnails list doesn't scroll to the right.

I've tried to fix it by calling actor.get_allocation_box() after creating the
ThumbnailList object as is already done on the show() method (but at that
point the ThumbnailList object isn't created yet).

This works better in that we now do scroll but isn't enough since we don't
really scroll all the way to the end. I've noticed that the allocation for the
highlighted thumbnail is wrong[1] when this occurs but I haven't been able to
actually fix that. I've run out of ideas so I'm filling this anyway in the
hope that someone who knows clutter better than I do can spot the problem
easily.

[1] allocation.x2 is shorter than it should. Checked with this log:

diff --git a/js/ui/altTab.js b/js/ui/altTab.js
index 59c6dd8..cd0a05e 100644
--- a/js/ui/altTab.js
+++ b/js/ui/altTab.js
@@ -666,6 +666,7 @@ SwitcherList.prototype = {
         let padding = this.actor.get_theme_node().get_horizontal_padding();
         let parentPadding =
         this.actor.get_parent().get_theme_node().get_horizontal_padding();
         let x = this._items[this._highlighted].allocation.x2 - monitor.width + padding + parentPadding;
+        log(this._items[this._highlighted].allocation.x2);
         Tweener.addTween(this._list, { anchor_x: x,
                                         time: POPUP_SCROLL_TIME,
                                         transition: 'easeOutQuad',