feat: customize target Device Pixel Ratios

[sherwinski]

Description

Related to https://github.com/imgix/js-core#fixed-image-rendering

Add a device_pixel_ratios option to the https://github.com/imgix/imgix-rb#fixed-image-rendering function to allow users to customize the target device pixel ratio(s) (dpr) when generating a fixed image. This will allow users to have more granular control over the fixed-size srcset values generated.

[jtarchie]

This is the line question we'd like to be able to configure. Will you accept a pull request? Please identify the interface you'd like too.

I want this to remove some targets to literally just save on bandwidth. We show a lot of images; this adds up.