account for hidden footers, no layout rounding, header track to use minmax

[wibjorn]

Fix bug where a hidden footer element would still register a height. Account for a few more small bugfixes too.

• Layout now always expects a header, but not necessarily a footer, since footers can be rendered within containers too.
• Stop accounting for rounding in calculations and remove -1px in the --atlas-contained-height property.
• Stop rounding in the measurements in JS.
• The header now has a track value of minmax(auto, max-content), which helps when switching between different layouts.

Testing

1. Visit page. http://localhost:1111/components/layout.html
2. There should be not up/down scroll on contained layouts on tablet +.
3. Scroll around, values of atlas-header-visible-height and atlas-footer-visible-height variables should update. When the respective elements go out of view.
4. If you put "display-none" on the footer and scroll or resize the header height vars should be zero.

Contributor checklist

• Did you update the description of this pull request with a review link and test steps?
• Did you submit a changeset? Changesets are required for all code changes.
• Does your pull request implement complex UI logic (js/ts)? Consider adding an integration test to test your user flow.
• Does your pull request add a new page to the documentation site? Add your new page for automated accessibility testing in /integration/tests/accessibility.

[changeset-bot]

🦋 Changeset detected

Latest commit: 21f2bcf

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@microsoft/atlas-css	Patch
@microsoft/atlas-js	Minor

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