This makes the cursor move smoothly left and right -- instead of
"stuttering" when passing over a zero-width character.
Pressing <Delete> on a normal (spacing) character also deletes
any zero-width characters after it. But pressing <Backspace>
while the cursor is placed after a zero-width character, just
deletes that zero-width character. The latter behavior allows
deleting and retyping just the combining diacritic of a character
instead of the whole character.
This addresses https://savannah.gnu.org/bugs/?50773.
Requested-by: Mike Frysinger <vapier@gentoo.org>
687efd210c