*This is a follow-up to PRs 6299 and 6441.*
The patch also adds an option to `PDFThumbnailView`, that disables the canvas-to-image conversion entirely, which might be useful in the context of issue 7026.
The `hasImage` property is a left-over from older thumbnail code, and has been made obsolete by `renderingState`.
Having two different properties tracking (basically) the same state is asking for trouble, since it's very easy to forget to update one of them, with annoying bugs as the result.
*Follow-up to PR 6299.*
This patch reduces unnecessary code duplication for the `canvas` to `image` conversion. It also does a bit of re-ordering (and adds new lines) in `_getPageDrawContext`, since that function currently is a bit hard to read.
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.)
