Allow multiple QGraphicsVideoItems to be displayed in one QGraphicsView on Symbian

Using the 'overlay' QGraphicsVideoItem implementation (src/multimedia/qgraphicsvideoitem_overlay.cpp), the native window handle of the QGraphicsView widget is passed to the backend via the QVideoWindowControl interface. This means that, if multiple QGraphicsVideoItems are displayed in a single QGraphicsView, they will all be given the same native window handle.

On Symbian, it is only possible to render one video to a given native window handle. Attempting to add a second QGraphicsVideoItem will therefore cause the first to stop displaying video and just appear black.

In order to allow multiple QGraphicsVideoItems to be displayed in the same view, each one must be given its own native window handle. This could be implemented as follows:

• QGraphicsVideoItem creates a parentless QWidget, and sets its z-order so that is appears behind the QGraphicsView window
• The area of the QGraphicsView window covered by the QGraphicsVideoItem is painted with transparency