Use a QImage instead of QPixmap in BackingStoreQt.

This is mainly to get rid of the warning shown when using QQuickWebEngineView
without delegated rendering with the threaded scene graph render loop:
"QPainter: It is not safe to use drawPixmap() outside the GUI thread"

The main advantage that QPixmap gave us was that QPixmap::scroll is
provided. This patch pulls out some of its implementation to make the
use case work with QImage.

Since QPixmap is backed by a QImage with the raster paint engine, the
previous behavior should remain except on the DirectFB platform, which
we don't ship to yet.

Change-Id: If77c9079ee95218519be1d092cad24635e29a279
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>