Vertical text creation

I needed to output vertical Japanese text, but was stymied by several limitations:

• No API to load a TTF as Identity-V encoding
• No support for 'vert' glyph substitution
• No support for vertical metrics ('vhea' and 'vmtx' tables are parsed but not used at all)

I have attached a series of patches that implement the above features. Highlights:

• The GSUB glyph substitution table is parsed (limitation: type 1 lookups only; this is sufficient for many features including 'vert'/'vrt2' vertical glyph substitution)
• Cmap lookup makes use of GSUB when features are enabled on a TTF
• 'vhea' and 'vmtx' metrics are applied to PDCIDFont when appropriate, and are embedded/subsetted correctly through the DW2/W2 CIDFont dictionary
• An API has been added for loading a TTF as a vertical font, setting Identity-V encoding and enabling 'vert'/'vrt2' substitution

Each patch could approximately be split out into a separate ticket, if desired.

Also attached is some sample code that exercises these patches and illustrates the effect of vertical glyph positioning. The sample output PDF is also attached.