[cairo] Should cairo metrics be returned in 'glyph space'?

[Keith Packard]

PostScript reutrns text metrics in 'glyph space', which is the unscaled 
'unit' square space in which glyph outlines are originally provided.

If we matched this in cairo, our text metrics adventures would be at an 
end.

If we don't, we need to figure out how to convert from 'glyph space' to 
'user space'.  Just running them through the font matrix "won't work" as 
the metrics aren't all vectors, so things like 'height' become meaningless 
without a glyph-centric coordinate space.

What I'm doing right now (to make things "work") is to extract only the 
scale portions from the glyph transformation and apply them to the metrics 
before returning them.  For scale-only transforms, this obviously works 
just fine.  But, any other transformation will be a bit confusing.

Another possibility is to create a vector datatype for the metrics 
generated from the glyph space metrics already computed.

-keith


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