[cairo] Compositing / blend functions documentation

[Bill Spitzak]

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.