Reuse the existing getOutputScale() machinery to render and/or rescale
thumbnail images to the native display resolution. Fixes blurry thumbnails on
HiDPI displays making it a lot easier to make out the content.
This includes an optimization to zero the height and width of existing thumbnail canvases, when they are removed and recreated during rotation of the document. (Credit goes to nnethercote, who initially found this in PR 4920.)
