[cairo] Various problems with using a recording surface

[Alexis Lamiable]

Hello,

I'm having some problems with using recording surfaces. I'm unsure
whether they are problems with the documentation, with the code, or with
the user :)

I used to simply draw on PNG or SVG surfaces, depending on what I
needed, but now I'm writing a library, and I'm trying to write clean
generic code. I want to draw small charts on recording surfaces, and
then redraw those surfaces on actual surfaces according to some layout.

First of all, the documentation says that I can pass NULL for the
extents parameter of cairo_recording_surface_create, in order to get an
unbounded recording surface. However, when I did that, I got no output
on my PNG surface. Here are excerpts from my code:

recording = cairo_recording_surface_create(CAIRO_CONTENT_COLOR_ALPHA,
NULL);
...
png = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, width, height);
cr = cairo_create(png);
cairo_set_source_surface(cr, recording, x, y);
cairo_paint(cr);
cairo_surface_write_to_png(cr, filename);

Replacing NULL by a (0, 0, width, height) rectangle makes it work, but
the documentation suggests that it's not necessary.

Secondy, I'm trying to do the same operations on SVG and PDF surfaces,
but this time I don't get anything displayable, with or without
boundaries. The only difference in my code is that I create another type
of surface, and then don't write_to_png. Am I doing something wrong? If
it's not me, I'll try to do a proper bug report with full code.

(All the objects are properly destroyed in the end, so it shouldn't be
due to a lack of flush/finish/destroy)

Thanks in advance for your help,
A. L.