Line canvas exclude method

[tznind]

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

This is a WIP. Current implementation only works for parallel exclusion, perpendicular exclusion and more tests will be added before I mark this ready. There may be 'off by 1' type bugs.

It is an alternative to the proposed DrawIncompleteFrame.

Use Case:
Ability to draw rectangles (or other shapes) with parts of the boundary line snipped out e.g. for TabView. Also in general, to apply an 'eraser' effect to a LineCanvas.

Approach:
Adds extension method Exclude to collections of StraightLine (this decouples the method from LineCanvas).

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

[tig]

Wow. Just wow. 🤩

[tznind]

Now works for perpendicular intersection too (cutting at most 1 cell out - i.e. splitting a line into 2).

[tznind]

One corner case bug currently is that the resulting lines are always of positive length.

Now a horizontal line x=1 with length 5 is the same as one with x=5 with length -5

But not so for the special case of length = 1 vs length = -1 when joined to another line. In this case a UL or LL is created depending on positivity of the length.

So I think I will need to ensure returned lines keep their +/- length.

See here when cutting horizontally just above the bottom line the bottom corners flip to pointing downwards (+1 length is down on y axis)

[BDisp]

One corner case bug currently is that the resulting lines are always of positive length.

No it can't, otherwise you can't control the flow.

Now a horizontal line x=1 with length 5 is the same as one with x=5 with length -5

That the right behavior.

But not so for the special case of length = 1 vs length = -1 when joined to another line. In this case a UL or LL is created depending on positivity of the length.

That was the problem I was facing before and I made confusion that corner could have more than one way to achieve them, but thankfully to you I learned that they must have unique values and the only exception is that no matter if vertical or horizontal are used first.

So I think I will need to ensure returned lines keep their +/- length.

Make sure it always return the signal value.

See here when cutting horizontally just above the bottom line the bottom corners flip to pointing downwards (+1 length is down on y axis)

Yes I see, but I think you can manipulate that by getting the Y position where you cutting and the corner case you are manipulating. If the desired effect is the opposite return the right ones.

[tznind]

Error is tree view colors unit test. Should be good to go though.