[cairo] “Conjoint-Over” Operator

[Lawrence D'Oliveiro]

Keith Packard published a description quite a while ago
<http://marc.info/?l=xfree-render&m=99792000027857&w=2> of additional
“conjoint” and “disjoint” variants of the traditional Porter-Duff
operators. This sample script
<https://github.com/ldo/python_pixman_examples/blob/master/conjoint_over>
illustrates the difference between the original Porter-Duff “over”
operator with the conjoint and disjoint variants, in the particular
situation that Keith was concerned with, namely accumulating increasing
opacity with multiple passes of a brush.

True enough, the conjoint-over version seems to give something closest
to the original colour, certainly in the earlier passes.