Fixes #2249. CharacterMap isn't refreshing well non-BMP code points on scroll.

[BDisp]

Fixes #2249 - Many of the non-BMP code point only occupies 1 column, but driver read 2 chars and advance 2 columns. The trick is back off 1 column before write next chars.

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

Pizza?

[tig]

Can you please target this at my PR #2612 instead of v2_develop?

As I indicated before I've made a tons of changes to the console drivers in #2612 and it will be very hard for me to merge new changes out of v2_develop. I'm working really hard on getting #2612 done so I can put it into v2_devleop but it will still be another week or so.

Thanks.

[BDisp]

Done @tig tig#13.