We originally defined Width to report in CSS px because the same header was responsible for reporting both the "intended image display width", or the "layout viewport size" for resources that don't have a "display width". However, we've since split these use cases (see #48) into separate headers: Viewport-Width and Width.

This update changes the definition of Width to report image display width (when it's known) in physical pixels, instead of CSS pixels. This offers a couple of important benefits:

<img srcset="wolf-480.jpg 480w, ..., wolf-1920.jpg 1920w" sizes="...">

1. The communicated value is aligned with w descriptor used for viewport-based selection.
2. The communicated value significantly improves interaction with HTTP caches:
   • With current definition the cache has to vary the response based on DPR and Width (i.e. Vary: DPR, Width) which leads to unnecessary fragmentation (see viewport-based selection use case and caching #59).
   • With updated definition the cache would have to vary on Width only.

Closes #59.