This patch intends to simplify future ES6 refactoring of the thumbnail code, since the current code isn't going to work well together with proper `Class`es.
Also replaces `var` with `let` in code that's touched in the patch. Please note that this should be completely safe, for two separate reasons, since trying to access let in a scope where it's not defined is first of all a runtime error and second of all an ESLint error (thanks to the `no-undef` rule).
In various viewer files, there's a number of cases where we basically duplicate the functionality of `createPromiseCapability` manually.
As far as I can tell, a couple of these cases have existed for a very long time, and notable even before the `createPromiseCapability` utility function existed.
Also, since we can write ES6 code now, the patch also replaces a couple of `bind` usages with arrow functions in code that's touched in the patch.
Note that as discussed on IRC, this makes the viewer slightly slower to load *only* in `gulp server` mode, however the difference seem slight enough that I think it will be fine.
*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.)
