wcwidth() is a POSIX function that returns the number of cells that a
wide-character occupies. The glibc function cannot be used as it depends
on the locale and we need _always_ UTF8 no matter what the locale is.
This implementation is provided by Markus Kuhn and is equivalent to
xterm's behavior.
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
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