[cairo] Text scaling and hinting

[Robert O'Callahan]

I'm currently working on having Mozilla draw text through the new Cairo font 
APIs (the text in my blog screenshot was a hack based on Xft). The new Cairo 
font APIs have all metrics depending on the current transformation matrix 
because of hinting effects. That's fine, but we also need the ability to get 
a cairo_scaled_font_t and render those exact outlines under other 
transformation matrices.

One obvious use case is animated rotation/scaling of a multi-line text 
object. If we allow the metrics to change during the animation, lines might 
break in different places during the animation, which is probably not what 
the author wants. Performance would also suffer because we'd have to run our 
layout algorithm at each and every step of the animation. Alternatively, we 
could just cross our fingers, hope that the metrics don't change much, and 
don't rerun the layout algorithm --- but that will lead to glyphs 
overlapping at the boundaries of text runs, and other ugliness.

What do you think?

Rob
-- 
["Therefore, my dear friends, as you have always obeyed–not only in my
presence, but now much more in my absence–continue to work out your
salvation with fear and trembling, for it is God who works in you to will
and to act according to his good purpose." Philippians 2:12-13.]
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