Add QPlatformBackingStore::toImage() implementations

On platforms where it makes sense, add the implementation that makes
returning a QImage possible.  This makes it possible to grab raster
windows.

Change-Id: I89241728aa6f7811c56bb66cd86a64aa961d71f8
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>