vertical writing: reference point of GlyphItem and text renderers?

Hi,

Vertical writing has been broken since we upgraded HarfBuzz we used to
1.4.8. ( To be more specific, the cause was introduced in HarfBuzz 1.3.4.
More detail can be found in [1] ). While I  saw x_offset and y_offset
returned from hb_buffer_get_glyph_positions in
vcl/source/gdi/CommonSalLayout.cxx changed between versions and caused the
issue, I didn't know what it meant.

Which point does maLinearPos of GlyphItem refer to? What is the origin of
the coordinate system it use when it comes to vertical writing? Do they
match the offest we get from harfbuzz?

I've also repored to HarfBuzz[2] but I will be appreciate if there is any
suggestion to solve this problem.

[1]https://bugs.documentfoundation.org/show_bug.cgi?id=111967
[2]https://github.com/behdad/harfbuzz/issues/573

Best Regards
-- 
Mark Hung