[cairo] API Shakeup: remove png backend

[Owen Taylor]

Carl Worth wrote:

>>   cairo_status_t
>>   cairo_image_surface_write_png (cairo_surface_t *surface,
>>                                  FILE            *file);

[..]

>>What error do we return if the surface isn't an image surface?
> 
> 
> CAIRO_STATUS_SURFACE_TYPE_MISMATCH ?

If we are willing to accept failure from write_png(), we could
make it cairo_surface_write_png(), use
_cairo_surface_acquire_source_image() and have it work for
X pixmaps, windows bitmaps, etc.

It obviously isn't going to work for PDF surfaces and similar, so it
would have to fail in some cases.

Regards,
					Owen