[cairo] Simplifying the operator set

[Carl Worth]

The recent discussions of the rendering equation have prompted me to
re-examine the list of compositing operators in cairo.

I'd like to simplify the operator set a bit, so I'm proposing the
following changes (roughly sorted with most trivial changes last):

* Eliminate CLEAR.

	This operator is a real oddball. It's the only one that does
	not examine the source pattern. And its result is easily
	emulated with SOURCE and a current alpha of 0.

* Eliminate ATOP, ATOP_REVERSE, and XOR.

	These operators are called "less useful" in the original
	Porter/Duff paper. I haven't seen any specific use for them,
	and they are easily decomposed into the union of other
	operators as described in the paper:

		"A atop B is the union of A in B and B out A"
		"A xor B is the union of A out B and B out A".

	Plus the name of the XOR operator has led several users to
	expect something entirely different, so eliminating that
	confusion source can only help.

* Abbreviate less: SRC -> SOURCE, DST -> DEST

	Cairo has an abbreviation-adverse naming style favoring
	legibility over dubious benefits such as keystrokes or file
	size [*]. "DESTINATION" might be pushing it though. We could
	use other names, (input and output?), which would deviate a
	bit further from RENDER, but not from Porter/Duff, (who only
	use A and B in the paper).

* Rename all of the REVERSE operators. FOO_REVERSE -> DEST_FOO

	These operators can be very useful, (it's fun to punch holes
	in things with OUT_REVERSE). But I am often have to do a long
	stop-and-think to figure them out. I think the mental
	gymnastics have to do with mapping "src OUT_REVERSE dest" to
	"dest OUT src". It seems a name of DEST_OUT will guide me more
	naturally to the correct operation.

* Reorder the list to group SOURCE and DEST.

So, if we do all of those, we end up with the following list:

	typedef enum cairo_operator {
	    CAIRO_OPERATOR_SOURCE
	    CAIRO_OPERATOR_OVER,
	    CAIRO_OPERATOR_IN,
	    CAIRO_OPERATOR_OUT,

	    CAIRO_OPERATOR_DEST,
	    CAIRO_OPERATOR_DEST_OVER,
	    CAIRO_OPERATOR_DEST_IN,
	    CAIRO_OPERATOR_DEST_OUT,

	    CAIRO_OPERATOR_ADD,
	    CAIRO_OPERATOR_SATURATE
	} cairo_operator_t;

Maybe that helps a bit, (or maybe it just doesn't matter that much).

But feedback is welcome, as always.

-Carl

[*] We do have some well-known acronyms that I don't mind at all,
    (rgb, pdf, ps, png).

    I note that there are a few abbreviations that have slipped into
    function names: "concat", "init", and "rel". These come from
    PostScript naming conventions, (though "rel" is already less
    abbreviated than PostScript's "r").