Previously, for a given point, we would create a char iterator at
the start of the row and the skip `column` characters. This is
however incorrect because display points are expressed in bytes,
and so we could park the anchor midway through a multi-byte character.
This commit fixes the issue by switching `DisplayMap::chars_at` to
take a point instead and skipping characters correctly when a point with
a non-zero column is provided.
Antonio Scandurra
d5ec0c8feb
