When the adjustment changes (due to e.g. a mouse wheel scroll) we update
the prelight. The part that un-prelights the previous prelight was
broken by the the pixel cache, as it called update_prelight in the
middle of the scrolling operation, where the windows were moved
but the tree_view->priv->dy was not changed to the new value. This
caused the updates to the pixel cache to go to the wrong place.

We fix this by fully doing the scroll before we update_prelight().

https://bugzilla.gnome.org/show_bug.cgi?id=728284