[cairo] Compositing / blend functions documentation

[Bill Spitzak]

Note that the Delphi document has got "A" and "B" swapped from the SVG 
and most other documentation.

Bill Spitzak wrote:
> Carl asked me where what documentation we had on blend functions.
> I asked around here for where we got the math and was pointed at this page:
> 
>     http://www.pegtop.net/delphi/blendmodes/
> 
> and also at the page Carl already referenced:
> 
>     http://www.w3.org/TR/SVG12/rendering.html
> 
> The SVG page describes the full functions including the effect of alpha. 
> The Delphi page ignores the alpha, but the functions can be converted: 
> if the simple function is F(A,B) and the alphas are a,b, the new 
> function is F(A/a,B/b)*ab+A(1-b)+B(1-a).
> 
> I think the Delphi page has some simpler and more likely functions for 
> some things, in particular softlight. It looks to me like the SVG guys 
> tried to figure out the function by testing it in PDF/Photoshop and 
> arrived at much more complex functions than necessary.
> 
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