The preview code still contained all the old logic for embedding the
preview into another widget. Most of the code had been dead for quite
some time.

The only preview rendering code now is the one to render into a pixmap.
This is actually changed to render the widget directly at the right
position obsoleting the previous widget positioning code.

The no preview available label is dropped as it did not get rendered into
the pixmap at all.

Last but not least we fake that compositing is on. This has always been
the case for Aurorae themes and just looks better. Especially on Oxygen I
had always had some corruption when rendering this preview with
compositing disabled.

REVIEW: 108777