Don't crash when Image.source change

QDeclarativePixmap::clear() may result in the texture being
scheduled for deletion, so we need to make sure that our texture
provider is updated properly whenever the source changes.

Change-Id: Ie21c6fcba4b0cd438250dc4c263b513060cc026e
Reviewed-on: http://codereview.qt-project.org/5673


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>