[cairo] Font metrics under font transformations

[Keith Packard]

Now that we have an answer for glyph metrics under font transformations, 
I'd like to see if anyone has an idea of what the font metrics should look 
like.

Font metrics include three vertical spacing values and maximum advance 
values in both directions (it's not a maximum advance vector as the values 
are compared separately).

Keeping our 'rotated glyphs are weird' view of the world, one simple idea 
would be to just return the 'scale only' metrics, ignoring the non-scale 
portion of the font matrix.  This will be hard to document, and will 
likely confuse anyone who does accidentally rotate a font.

An alternative would be to compute the vertical line spacing in user space 
between adjacent lines; this works except for 90° rotations where it 
produces an infinite result.  We could return 'inf/-inf' in this case if 
necessary.

The third option is to return vectors for all of these values.  This will 
complicated usage of these values in the 'normal' cases, but will 
otherwise work just fine.  Note that 'max_x_advance' and 'max_y_advance' 
will *also* be vectors as they point along the font space baseline.

-keith


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