Tighter bounding box check

Tighter bounding box check

Because the gernic check works on floats and how .toPoint rounds, we
need to redo the bounds checking inside Turbulence

Task-number: QTBUG-21564
Change-Id: Ib56a8d420d9abf8035360b7908e89e28938799a8
Reviewed-on: http://codereview.qt-project.org/5343
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>