[cairo] Font metrics under font transformations

[Keith Packard]

Around 21 o'clock on Jan 11, Maarten Breddels wrote:

> No matter how weird the transformation is, the 'add advance vector' and 
> 'substract height vector' should always work, i think.

I've just thought of an obvious reason we can't just run the height 
through the font transformation and get a vector result.  Artificial 
obliquing uses a shear transformation:

	font space		user space
	[x,y]		-> 	[x + y * shear_factor, y]

Now run our simple [0,height] vector through this transformation to 
compute the 'line spacing vector':

	[0,height]	->	[height * shear_factor, height]

I submit that the line spacing should remain constant under a shear 
transformation.

Yuck. Suggestions anyone?

-keith


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