LineCanvas support for mixing double and single lines

[tznind]

Fixes #_____ - Include a terse summary of the change or which issue is fixed.

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tznind]

Mixed line style support, have doubles merge into singles and vice versa!

On my laptop on holiday so can't do the Ctrl+K+D thing but otherwise should be good to go.

[tig]

See comments. It's not working for all orientations.

[tig]

Also, @tznind, can you please move LineCanvasTests.cs to the Core folder and ensure the namespace is Terminal.Gui.Core?

[tznind]

Also, @tznind, can you please move LineCanvasTests.cs to the Core folder and ensure the namespace is Terminal.Gui.Core?

Done. Guessing you mean Terminal.Gui.CoreTests though right? Not justTerminal.Gui.Core?

[tig]

I just grabbed your PR again using a different technique and now it's correct and working correctly.

git remote add tznind tig:tznind/gui.cs.git
git fetch tznind
git checkout -b tznind-line-canvas-style-mixing tznind/line-canvas-style-mixing  

😕

[tig]

<3

[tznind]

@tig just recording this here as I don't think theres much we can do about it but the current implementation still looks a bit janky when changing from double to single 'mid rune' along a specific axis. For example if you draw 2 boxes, one with single width and one with double width overlapping: