In some cases PDF documents can contain JPEG images that the native browser decoder cannot handle, e.g. images with DNL (Define Number of Lines) markers or images where the SOF (Start of Frame) marker contains a wildly incorrect `scanLines` parameter.
Currently, for "simple" JPEG images, we're relying on native image decoding to *fail* before falling back to the implementation in `src/core/jpg.js`. In some cases, note e.g. issue 10880, the native image decoder doesn't outright fail and thus some images may not render.
In an attempt to improve the current situation, this patch adds additional validation of the JPEG image SOF data to force the use of `src/core/jpg.js` directly in cases where the native JPEG decoder cannot be trusted to do the right thing.
The only way to implement this is unfortunately to parse the *beginning* of the JPEG image data, looking for a SOF marker. To limit the impact of this extra parsing, the result is cached on the `JpegStream` instance and this code is only run for images which passed all of the pre-existing "can the JPEG image be natively rendered and/or decoded" checks.
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*Slightly off-topic:* Working on this *really* makes me start questioning if native rendering/decoding of JPEG images is actually a good idea.
There's certain kinds of JPEG images not supported natively, and all of the validation which is now necessary isn't "free". At this point, in the `NativeImageDecoder`, we're having to check for certain properties in the image dictionary, parse the `ColorSpace`, and finally read the actual image data to find the SOF marker.
Furthermore, we cannot just send the image to the main-thread and be done in the "JpegStream" case, but we also need to wait for rendering to complete (or fail) before continuing with other parsing.
In the "JpegDecode" case we're even having to parse part of the image on the main-thread, which seems completely at odds with the principle of doing all heavy parsing in the Worker, and there's also a couple of potentially large (temporary) allocations/copies of TypedArray data involved as well.
In order to eventually get rid of SystemJS and start using native `import`s instead, we'll need to provide "complete" file identifiers since otherwise there'll be MIME type errors when attempting to use `import`.
Note that Prettier, purposely, has only limited [configuration options](https://prettier.io/docs/en/options.html). The configuration file is based on [the one in `mozilla central`](https://searchfox.org/mozilla-central/source/.prettierrc) with just a few additions (to avoid future breakage if the defaults ever changes).
Prettier is being used for a couple of reasons:
- To be consistent with `mozilla-central`, where Prettier is already in use across the tree.
- To ensure a *consistent* coding style everywhere, which is automatically enforced during linting (since Prettier is used as an ESLint plugin). This thus ends "all" formatting disussions once and for all, removing the need for review comments on most stylistic matters.
Many ESLint options are now redundant, and I've tried my best to remove all the now unnecessary options (but I may have missed some).
Note also that since Prettier considers the `printWidth` option as a guide, rather than a hard rule, this patch resorts to a small hack in the ESLint config to ensure that *comments* won't become too long.
*Please note:* This patch is generated automatically, by appending the `--fix` argument to the ESLint call used in the `gulp lint` task. It will thus require some additional clean-up, which will be done in a *separate* commit.
(On a more personal note, I'll readily admit that some of the changes Prettier makes are *extremely* ugly. However, in the name of consistency we'll probably have to live with that.)
