Aurorae: Add dummy window for working device pixel ratio

Aurorae uses a QQuickWindow that receives an implicit scale through the
QPA. Unfortunately there's a known quirk with device pixel ratio not
being followed when offscreen surfaces are never shown and an
independent render control is used.

Without this QQuickRenderControl::grab ends up using a buffer the size
of the window * dpr, but with the contents rendered at 1x in the corner
along with incorrect device pixel ratio information.

This patch copies a used solution from EffectQuickView. This can be
resolved more neatly with newer Qt.

BUG: 452730
BUG: 465790