[Text] Convert control codepoints into zero width space during shaping

[Gillibald]

What does the pull request do?

This PR changes the current text shaper implementation, transforming non-shapable control codepoints into zero-width space glyphs. So they are not visible in the shaped result.

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

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051858-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

I might not understand reasoning behind this, but isn't it common to render � for unknown characters? Making it easier to notice problem with a font or text.

[Gillibald]

This is not about unknown characters. These are still shaped as a replacement glyph. We are just replacing codepoint that have the general category of control. These are usually not visible.