Keep a skin-toned emoji in one two-column cell. Issue 13079

A skin tone modifier after an Emoji_Modifier_Base no longer starts its
own cell, and the resulting emoji modifier sequence is double-width.
A modifier after anything else still gets its own cell, since CoreText
draws it as a separate swatch. The preconverted-string fast path now
segments the same way StringToScreenChars does so a skin tone arriving
in a later write is merged into its base rather than written twice.