[cairo] separating compositing and blending

[Rik Cabanier]

All,

the current cairo compositing operators combine blending and compositing
[1].
This means that if you do an 'overlay', the result of the 'overlay'
blending formula will always be composited with 'src-over'.

Nikos (from Canon) and I (from Adobe) have a proposed working draft
that allows people to specify blending and compositing in CSS [2]. As
an example, this would allow you to do a 'screen' blend on an HTML
element with a 'src' compositing mode. This separation could also be
extended to canvas.

Looking at the Cairo architecture, it seems that this could be added
by adding an optional blend mode operator to 'set_operator'.
What do people think?

Rik

1: http://cairographics.org/operators/
2: https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html
3: http://cairographics.org/manual/cairo-cairo-t.html#cairo-set-operator
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