Added PerformanceHints flags to QSGPaintedItem.

Added PerformanceHints flags to QSGPaintedItem.

At the moment only contains FastFBOResizing.
If this flag is set to true and when using a FBO as render target,
the FBO will use a larger texture than the size of the item to avoid
too many resizing.

Change-Id: I3b8a51a5a07329ff3ed010a35dc8235913201a8e
Reviewed-on: http://codereview.qt-project.org/4651
Reviewed-by: Yoann Lopes <yoann.lopes@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>