Handle Exif orientations for JPEG images (fixes #1670)

[prewett-toptal]

Implements simple Exif parsing for JPEG files, enough to determine if there is an Exif tag, and if that tag has an orientation, rotates the image. This makes node-canvas compatible with browsers, which also rotate the image based on the Exif orientation tag. This fixes #1670.

A simple test program for 2 of the 3 paths follows. This loads the JPEG, draws it to the canvas, and saves the result to ./test.png. Test images are available (among other places) from https://github.com/noell/jpg-exif-test-images:

`
const fs = require('fs')
const { createCanvas, loadImage } = require('canvas')

// One source of sample images with EXIF rotations is at
// https://github.com/noell/jpg-exif-test-images

const imagePath = process.argv[2];
loadImage(imagePath).then(image => {
const w = image.naturalWidth
const h = image.naturalHeight
const canvas = createCanvas(w, h/, 'PDF'/) // uncomment to test the mime device path
const context = canvas.getContext('2d')
context.fillStyle = '#f0f';
context.fillRect(0, 0, w, h);
context.drawImage(image, 0, 0, w, h)

const buffer = canvas.toBuffer('image/png')
fs.writeFileSync('./test.png', buffer)

}).catch(e => {
console.log("[error]", e);
})
`

Since one major use of node-canvas is in the fabric project, which has not updated to the newly release 3.0 version yet, I think it would be helpful to backport to 2.11 (they use 2.11.2 at the moment), since it seems that 3.0 makes breaking changes for fabric. I initially made these changes on a branch from the v2.11.2 tag, and they merged with no problems, so I expect this changeset can be merged into a 2.11 branch. I can also create a separate pull request to whatever branch would be appropriate, if that is helpful.

• Have you updated CHANGELOG.md?

[prewett-toptal]

Would it be possible to get a yes/no on this? Thanks!

[chearon]

This looks really good, I definitely think we want this! Review time has been scarce for a while, but I'm carving out more time and I should be able to do a review soon.

[chearon]

Looks great. I don't understand why libjpeg doesn't read out exif info but I'm glad this doesn't introduce a dependency.

I tested the images you linked to and they all worked. I'll open a separate PR that adds those.

Some of of this is a bit @zbjornson's wheelhouse (pixel copying) so I might wait a bit for other comments before merging.

[chearon]

Can you move this to 3.0.0 below?

[prewett-toptal]

Done

[chearon]

Should this be doing a bounds check? I see that you're always calling hasBytes before getNext, but wonder if it's better for future changes to guard this. getc below essentially does bounds checking, I think.

[prewett-toptal]

Personally, I think there's no point doing unnecessary bounds checking, but I've added it.

[chearon]

I think it would be helpful to backport to 2.11 (they use 2.11.2 at the moment), since it seems that 3.0 makes breaking changes for fabric. I initially made these changes on a branch from the v2.11.2 tag, and they merged with no problems, so I expect this changeset can be merged into a 2.11 branch. I can also create a separate pull request to whatever branch would be appropriate, if that is helpful.

Can you link to any issues that describe the 3.0/fabricjs problem? I'm not sure how easy backporting will be since we've changed prebuilds drastically for 3.0.0.

[prewett-toptal]

I am not aware of any issues with Fabric and node-canvas 3.0. It would just be nice to have this in (our) production, and as I understand that 3.0 is a bit of an API change, perhaps Fabric is in no hurry to update.

[prewett-toptal]

(Regarding why libjpeg does not perform EXIF rotation, I think it's because the library is so old, but since it is used in so many locations they don't want to change anything. libjpeg-turbo, which I think is more commonly used (due to better performance), has rejected implementing EXIF rotation, for among other reasons, compatibility with libjpeg, since it aims to be a drop-in replacement, just faster. libjpeg-turbo is also an ISO reference implementation for the JPEG spec, so in some sense, it is libjpeg these days. Anyway, you can read their justification for refusing to implement EXIF rotation at libjpeg-turbo/libjpeg-turbo#774, and the issue that references. But it is a sorry state of affairs.)