[cairo] Applying to a surface the alpha of other

[Krzysztof Kosiński]

2012/10/22 Bill Spitzak <spitzak at gmail.com>:
> He does not want to un-premultiply.

I'm not so sure about this. Without un-premultiplication, the result
is equivalent to painting the image with CAIRO_OPERATOR_OVER onto a
black surface. With un-premultiplication, the result is as if the
alpha was reset to zero while keeping the color intact.

Regards, Krzysztof