Problem: Unpaired marks are shown with `filter.extmarks == true`, which
should only return visible highlights. Misleading `end_col`
included in `inspect_pos()` for unpaired mark; it is set to
`start_col + 1` which would be a visible highlight, which it is
not. Custom "is_here" filter used to get extmarks overlapping a
position.
Solution: Exclude unpaired highlight extmarks with `filter.extmarks == true`.
Set `end_col` to `start_col` for an unpaired mark. Supply
appropriate arguments to nvim_buf_get_extmarks() to return
overlapping extmarks; exclude marks whose end is at `{row, col}`
with `filter.extmarks == true`.
d40481322a