Pass a persistent QImage to QPixmap::fromImage when paint engine is raster.

For native engines, a temporary QImage can be provided, but raster will just take
and store the QImage, so a memcpy is needed.
The paint engine relevant for pixmaps can be detected by opening a painter on a
1x1 pixmap and querying the QPaintEngine. For optimization, this is done only once
per application lifetime because DImg::convertToPixmap is very time critical.

BUG: 278320