[WIP] Distribute LineGap evenly to top and bottom

[Gillibald]

What does the pull request do?

The PR adjusts the baseline calculation so that a defined line gap is distributed evenly to the top and bottom of the text line

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation
• Add a test for GlyphRun.BaselineOrigin
• Add a test for TextLine.Baseline
• Add a test for ShapedTextRun.Baseline

Breaking changes

Obsoletions / Deprecations

Fixed issues

Fixes: #19547

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0058619-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

I'm sorry to be that guy again, but please add a test so we can validate the calculated behavior and we won't regress it in the future.

[Gillibald]

• Add a test for GlyphRun.BaselineOrigin
• Add a test for TextLine.Baseline
• Add a test for ShapedTextRun.Baseline

[Gillibald]

We need to find a font file that defines typoLineGap or hheaLineGap otherwise, we can only run the tests on Windows

[miloush]

We can create some or edit the current test fonts as needed

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0058704-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]