[cairo] Failing to build in image-backend-only configuration

[Kalle Vahlman]

Hi!

I tried to build cairo with only the image backend (and the implied
font backend) to see what's the absolute minimum set of dependancies
for Cairo, but the build fails since the cairoint.h header refers to
cairo_surface_write_to_png_stream which is obviously not present since
cairo-png.c is not included.

Now, this is might look a clear issue at first, but actually
cairo-png.c is not a part of the png surface, but is an utility thing.

So the question becomes, should the --without-png be interpeted as "no
png at all please" (and thus the slim_hidden_proto-thing fixed to not
include the above symbol) or should it be just "no png surface please"
(and the build fixed to enable the cairo-png.c if libpng is available
and offer an option to disable it).

The third case of course would be to always require libpng, but I
would hate to see libpng as a hard dependancy just because an utility
API uses it...

-- 
Kalle Vahlman, zuh at iki.fi
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