Support device-pixel-ratio for cli-snapshot

[samarsault]

The problem

Resources with higher pixel densities are not capture when using along with dom snapshot i.e. in case of cli-snapshot,

Environment

• @percy/cli version: 1.18
• Version of Percy SDK you’re using:
• If needed, a build or snapshot ID:
• OS version:
• Type of shell command-line [interface]:

Details

If necessary, describe the problem you have been experiencing in more detail.

Code to reproduce issue

snapshot.yml

- name: DPR 
  url: https://mdn.github.io/learning-area/html/multimedia-and-embedding/responsive-images/srcset-resolutions.html
  widths: [475]
  discovery:
    device-pixel-ratio: 3

npx percy --verbose snapshot snapshot.yml