Fix gvRenderData bytes output

[Nagael]

This PR fixes the AGraph._draw(format='png') case. The PNG format can contain NULL bytes, so this call could result in an assertion error:

File "[...]/pygraphviz/agraph.py", line 1729, in _draw
    assert len(dot_string) == length
AssertionError

This fix correctly uses SWIG's %cstring_output_allocate_size(parm, szparm, release) instead of %cstring_output_allocate(parm, release) (as explained in 11.3.4 of https://www.swig.org/Doc4.0/Library.html)

Unfortunately my local SWIG installation is 4.0.1, so the regenerated SWIG files are more different than strictly necessary. The first commit is the main one, though.

[rossbar]

Thanks @Nagael , this seems good. The one missing piece is a test that illustrates the failure; i.e. something that fails with the current code that passes with this patch. Do you have any example cases where you hit this problem that could be added to the test suite?

[Nagael]

Thanks for your answer. I just added a commit with a test that fails on the current version. It asks for the png format, and at first I thought that the test should not assume that this format is available. But since other tests also use this format, I removed the check in a second commit. You can use whichever version is more convenient for the project.

[codecov-commenter]

Codecov Report

Merging #424 (1dcb195) into main (8d15432) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #424      +/-   ##
==========================================
- Coverage   80.87%   80.86%   -0.02%     
==========================================
  Files           5        5              
  Lines        1255     1254       -1     
==========================================
- Hits         1015     1014       -1     
  Misses        240      240              

Impacted Files	Coverage Δ
pygraphviz/agraph.py	81.29% <100.00%> (-0.02%)	⬇️
pygraphviz/graphviz.py	76.53% <100.00%> (ø)

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[rossbar]

Thanks for this @Nagael . I went ahead and re-generated graphviz.py and graphviz_wrap.c with SWIG 4.0.2. I'm not too familiar with SWIG, so if you wouldn't mind taking a quick look at the generated graphviz_wrap.c code to verify that everything looks as expected that'd be great.

I also went ahead and renamed/added a comment to the test so we have a better idea of what it's designed to catch. If you have any wording/name changes you think would be better, please go ahead and make them. Other than that, this LGTM! We always squash-merge so no need to worry about cleaning up the history.

[Nagael]

Thanks @rossbar, everything looks just as expected and works smoothly. Looks good to me too!

[rossbar]

Great, then in this goes (the failing CI is a known issue). Thanks again @Nagael !