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.