Would a `Val::PhysicalPx` be useful?

## What problem does this solve or what need does it fill?

`Val::Px` represents a length in logical pixels.
`Val::PhysicalPx` (or whatever name it was given) would represent a length in physical pixels.
It would be easy to implement, but I'm not sure how useful it would be.

No files, tests, or entry points are named. Start by reviewing how Val::Px is used and how logical and physical pixels are handled, then determine whether a separate Val::PhysicalPx is useful and what API behavior it should have. Done means reaching a clear decision on the feature's usefulness and scope.

Written by the indexing model from the issue text.