`HtmlInline` (and possibly other inlines) incorrect end line calculation

[SamWilsn]

It seems like the sourcepos calculation for inlines (at least for HtmlInline) adds self.line_offset to the end column, instead of the line offset for the end line.

For example:

  hello <img
    src="foo"
    alt="bar"> world

Gives a sourcepos of 1:9-3:9, while:

  hello <img
    src="foo"
                              alt="bar"> world

Also gives a sourcepos of 1:9-3:9.

[kivikakk]

(As background: indeed — sourcepos for inlines is not actually generally supported! Hence the support enabled in rendering with experimental_inline_sourcepos. @digitalmoksha has done a lot of good work on getting them more useful, but the upstream reference implementation doesn't support them at all, so Comrak's support has only recently been growing in this area.)