Reduce duplication in `dpi` module

[madsmtm]

Grabbed from https://github.com/rust-windowing/winit/pull/1374. The motivation in here was to add a Delta variant of these (as part of a replacement of MouseScrollDelta).

The only API changes in this PR is the PartialOrd/Ord impls, as noted in the changelog, the rest is merely a code-reduction that I believe is valuable (makes it much easier to see what's different between Size and Position).

Tagging original author @Osspial.

• [x] Tested on all platforms changed
• [x] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [x] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [x] Created or updated an example program if it would help users understand this functionality
• [x] Updated feature matrix, if new features were added or implemented