Make translucent windows working properly with OpenVG.

The OpenVG engine correctly uses vgClear() to fill the surface with
transparent pixels whenever the window has the WA_TranslucentBackground
attribute enabled. However both scissoring and masking affects
the operation of vgClear(). Drawing artifacts were previously
visible due this, simply because scissoring was left enabled by
the VG paint engine, and the filling with transparent pixels
happens in the window surface's beginPaint() that is called
between the paint engine's end() (for the previous paint) and
begin() (for the next paint).

Task-number: QT-4907
Reviewed-by: Jani Hautakangas

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