Fix order of QSGItem mouse filtering.

Fix order of QSGItem mouse filtering.

QGraphicsView filtered child items beginning with the item's
parent, grandparent, greatgrandparent... QSGCanvas did the
opposite, which breaks the QML mouse handling element filtering logic.

Task-number: QTBUG-21446
Change-Id: I18e125305eef536237195895a7f41f88b532d4aa
Reviewed-on: http://codereview.qt-project.org/5819
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Bea Lam <bea.lam@nokia.com>