[Cairo] Text APIs round 2

[Keith Packard]

Around 13 o'clock on Aug 15, Carl Worth wrote:

> Is the concern then for overlapping glyphs from separate fonts that
> need to be pre-composited together? Is that commonly needed? Or is
> there some other aspect of "correct" output I'm not thinking of?

Yes, that's the rendering concern.  When using modes like 'in', it's kinda 
important that all of the glyphs get drawn at the same time...

> I'm fine accepting the font-per-glyph API if there's a compelling need
> for it. (The number of backend text requests generated seems an
> optimization issue that could still be handled without requiring this
> API).

Attempting to merge text requests together inside the library is pretty 
hard to manage unless we add a 'text_flush' function, and I don't think we 
want that.

> In the absence of a compelling need, I would prefer not including the
> font in the glyph structure --- that would seem a better fit for the
> stateful nature of the rest of Cairo.

Not providing it means that some things will be very awkward and require a 
temporary surface.  I guess I could live with things either way though.

-keith