[XeTeX] VORG table and vertical writing support

[Jjgod Jiang]

Hi,

As described in [1], OpenType Spec 1.3 added VORG table, when this table
is present, the y coordinate of a glyph's vertical origin can be calculated
directly from the 'VORG'. XeTeX doesn't seemed to support this. Plus, in
the vmtx table, it seems only advanceHeight is used, topSideBearing of each
glyph is totally ignored, any reason for that?

- Jiang

[1] http://www.microsoft.com/typography/otspec/VORG.htm