Reduce compile warnings

Also change QSGDirection::sample to pass by value.

Change-Id: I878649158b78da22afdf77b5f3ce3263ce1070cd
Reviewed-on: http://codereview.qt-project.org/4497


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>