[PDFThumbnailView] Re-factor the `canvas` to `image` conversion such that we always render to a `canvas`, and then replace it with an `image` once rendering is done
Functionality wise, `ensureThumbnailVisible` is essentially just a shorthand for `scrollThumbnailIntoView`. (And note that `PDFViewer` doesn't implement a `ensurePageVisible` method.)
The only remaining usage of `PDFThumbnailViewer_ensureThumbnailVisible` is inside `PDFPresentationMode`, which introduces an otherwise unnecessary `PDFThumbnailViewer` dependency there.
We're already relying on the `presentationmodechanged` event, in various files, to track the state of Presentation Mode. Thus we can simply listen for that event in `PDFSidebar` too, and update the thumbnails if necessary.
*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.
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.)
