Don't change character set if unnecessary

[KazDragon]

If a character would be written in a different character, but it's actually the same character (e.g. 'X' is both US_ASCII and UK), then don't bother changing the character sets.

[KazDragon]

This would involve having a database of characters and what entries in character sets correspond to what characters.

[KazDragon]

An easy half-way solution would be to avoid character set switches for character 0x00-0x7F, which are the same in all character sets.

[KazDragon]

Note: DEC graphical characters has a smaller overlapping character set.

[KazDragon]

This may be required for anything complicated using Windows Terminal, see microsoft/terminal#16529