[BASE] Interpretation of vertical baselines

[behdad]

With @matthiasclasen we've been busy this week using BASE table API's in HarfBuzz in Pango to implement baseline alignment when fonts / scripts change within a line. A big confusion came up within vertical text. To quote harfbuzz/harfbuzz#3447

> The OpenType spec says this about the BASE table:
> 
> ```
> Each baseline coordinate is defined as a single X or Y value in design units measured from the zero position on the relevant X or Y axis.
> ```

But it doesn't actually say what the zero position for the axes is.

It would be plausible to use the origin of vertical glyph metrics for vertical baselines (which would put the central baseline at 0).

Indeed, given that glyphs in vertical writing are positioned according to their vertical origin, it only makes sense to me if the baselines for vertical writing are relative to the central baseline. But inspecting Noto Sans CJK suggests that they are not. So we are really confused.