[cairo] Feature request: Additional Blendmodes

[Franz Schmid]

Hi,

i'm Franz Schmid from the Scribus Team, and i've tested the new
group push and pop functions of your latest cairo snapshot to
achieve layer transparency in Scribus.

Congratulations it works perfect, saves me about 30 lines of code
and is much faster than my implementation with 2 image surfaces.

So far so good, but in order to be able to get the real transparency
groups as PDF-1.4 provides we need more blendmodes. The existing
Porter and Duff blendmodes are fairly useless if you want to emulate
the PDF-1.4 transparency modes.

It would be very, very fine if you could implement the blendmodes
that are mentioned in the PDF-Reference page 491 and following.

These are: Multiply, Screen, Overlay, Lighten, ColorDodge,
            ColorBurn, HardLight, SoftLight, Difference,
            Exclusion, Hue, Saturation, Color and Luminosity

With these modes in place we could make Scribus getting a bit closer
to the big marketleaders.

Franz Schmid
Scribus-Team