[Cairo] PNG backend for cairo?

[Carl Worth]

As most of you know, I rarely use cairo for more than trivially simple
demonstrations. But, since I'm getting pretty good at those, I'm
tempted to make them even more trivial.

In order to do so, I'm tempted to add a PNG backend to cairo. (I
actually think this would be helpful beyond the case of trivial demos,
but I can't prove that since I almost never write those kinds of
programs).

The problem with adding a PNG backend is that it would introduce a new
dependency, (on libpng), and of course I'd like to minimize
dependencies.

One option is to introduce a new helper library libcairo-png or some
such with its own <cairo-png.h>. This is possible since the PNG
backend only requires functionality already in the public API. This is
in contrast to the X/Render backend or a potential PDF backend, which
require internal functionality.

But I don't like the notion of confusing the user with respect to
which backends are builtin and which are available only in helper
libraries.

So, I'm back to wanting to shove the PNG backend directly into
cairo. While some might see the libpng dependency as gratuitous, some
might already find the X library dependencies equally (or more)
gratuitous. So maybe all we need is the ability to optionally disable
backends at compile time?

Any thoughts to help me out?

-Carl

PS. Here's a fairly minimal, but complete example possible with the
proposed PNG backend. The differences compared to what's possible
today is that there's no need to allocate an image buffer, worry about
stride, or write code to convert the image and write it out as a PNG.

#include <cairo.h>

int main (void)
{
    cairo_t *cr;

    cr = cairo_create ();

    cairo_set_target_png (cr, "demo.png", CAIRO_FORMAT_ARGB32, 100, 100);

    cairo_move_to (cr, 10., 10.);
    cairo_line_to (cr, 90., 90.);
    cairo_set_rgb_color (cr, 0., 0., 1.);
    cairo_fill (cr);

    cairo_destroy (cr);

    return 0;
}