[cairo] API Shakeup: Simplifying the operator set

[Carl Worth]

As part if the API Shakeup, I'd like to renew a less-aggressive version
of my earlier proposal to simplify the operator set. I now propose:

	typedef enum cairo_operator {
	    CAIRO_OPERATOR_CLEAR,

	    CAIRO_OPERATOR_SOURCE,
	    CAIRO_OPERATOR_OVER,
	    CAIRO_OPERATOR_IN,
	    CAIRO_OPERATOR_OUT,
	    CAIRO_OPERATOR_ATOP,

	    CAIRO_OPERATOR_DEST,
	    CAIRO_OPERATOR_DEST_OVER,
	    CAIRO_OPERATOR_DEST_IN,
	    CAIRO_OPERATOR_DEST_OUT,
	    CAIRO_OPERATOR_DEST_ATOP,

	    CAIRO_OPERATOR_XOR,
	    CAIRO_OPERATOR_ADD,
	    CAIRO_OPERATOR_SATURATE
	} cairo_operator_t;

As before, the primary change here is in naming convention. SRC is
expanded to SOURCE and DST to DEST. (Keith put his foot down before I
could expand that to DESTINATION).

The OP_REVERSE convention is also changed to DEST_OP and these are
re-ordered to group all SOURCE and DEST operators, (which hopefully
makes them easier to learn).

Unlike my earlier proposal, this does not eliminate any of the
Porter/Duff operators that I had nominated for the axe before, (Owen
recently found a good use for ATOP). And Keith makes a fairly
compelling argument in favor of algebraic completeness, (even the
no-op CAIRO_OPERATOR_DEST might come in handy as a graphical analog of
#if 0, who knows?).

I've decided against trying to invent new, more intuitive names for
existing Porter/Duff operators. While in some cases there are
reasonable alternative names, I don't think we could be equally
successful across the board, so I think we'd end up just being
different without much benefit.

Also, names like DEST_OVER (as opposed to say, UNDER), will receive
reinforcement from other systems that have adopted a similar
convention, (eg. "dst-over" in SVG).

We still do have room to add other, "fancier", compositing operators
in the future.

-Carl


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