feat: append variable qualities to dpr srcsets

[sherwinski]

This library will automatically append a variable q parameter mapped to each dpr parameter when generating a fixed-image srcset. This technique is commonly used to compensate for the increased filesize of high-DPR images. Since high-DPR images are displayed at a higher pixel density on devices, image quality can be lowered to reduce overall filesize without sacrificing perceived visual quality. For more information and examples of this technique in action, see this blog post.

This behavior will respect any overriding q value passed in as a parameter. Additionally, it can be disabled altogether by passing { disableVariableQuality: true } to the third argument of buildSrcSet().

The following map of q to dpr pairs will be generated:

dpr	q
1	75
2	50
3	35
4	23
5	20

This behavior specifically occurs when a fixed-size image is rendered, for example:

var srcset = new ImgixClient({
          domain: 'testing.imgix.net',
          includeLibraryParam: false
        }).buildSrcSet('image.jpg', {w:100});

will generate a srcset with the following q to dpr mapping:

https://testing.imgix.net/image.jpg?w=100&dpr=1&q=75 1x,
https://testing.imgix.net/image.jpg?w=100&dpr=2&q=50 2x,
https://testing.imgix.net/image.jpg?w=100&dpr=3&q=35 3x,
https://testing.imgix.net/image.jpg?w=100&dpr=4&q=23 4x,
https://testing.imgix.net/image.jpg?w=100&dpr=5&q=20 5x

[ericdeansanchez]

🎉 This PR is included in version 0.3.0-beta.1 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

Your semantic-release bot 📦🚀