[CREATE] OpenRaster spec: proposal: add svg:src-atop and svg:dst-atop

[Andrew Chadwick]

Hi, CREATE-friends --

We'd like to add support for the "Destination Atop" and (especially)
"Source Atop" compositing modes in MyPaint, to allow users to make
groups clipped by their bottom layer's alpha. Use case: add shadows
and highlighting without having to create a separate masking layer set
to dst-in/dst-out.

https://github.com/mypaint/mypaint/pull/284
https://github.com/mypaint/mypaint/issues/181#issuecomment-89778738

http://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_srcatop
http://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_dstatop

Does anyone have any objection to "svg:src-atop" and "svg:dst-atop"
being added to the list at:

https://wiki.freedesktop.org/www/Specifications/OpenRaster/Draft/LayersStack/#composite-opattribute

The blending function for the modes is to be Normal.

I'm Cc:ing Boud because I bet Krita already has this... Hopefully the
naming of the element won't be an issue; the pattern has always been
fairly obvious.

-- 
Andrew Chadwick