Preview image is returned when a 2D item render results in a blank
image.
Also improved device pixel ratio handling. No longer always render
preview at 2x ratio, if not needed.
Change-Id: I67b7563dffea523ea85be23b6a0e8b802c0f03fb
Fixes: QDS-2893
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>
This quietens warnings from libPNG during startup and shrinks the
image sizes.
Change-Id: Ieb4cb5e8ba30b99653896e283c2fb2cc267257f2
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
