Strange behaviour on arc drawing with big line width

Hello,
I'm writing a cairo python script, which loads graphics and animations from
a custom file to create "fancy" (and unuseful) graphics like those seen in
videogames and scifi movies.
I have a problem with arcs, though: I need to be able to draw them
interactively, with user given coords, start/end and line width. When I try
to draw arcs with a certain line width I have a strange behaviour: the
beginning of the line is not straight as it should be, you can see an
example of it here:
http://jidesk.net/sypy/strangearc2.jpg

Here is the code I used for this test:
https://bpaste.net/show/e1f3b8d1f842

I'm using cairo 1.12.16 from the official Gentoo portage (the problem still
remains with the latest -r3 version in the portage), with
pycairo 1.10.0-r4, also latest stable from the portage.

CC'ing to [email protected] who confirmed the issue on #cairo using
cairo 1.13.0~20140204-0ubuntu1, but was not present using
cairo  1.12.16-0ubuntu1.1~bryce~precise6, with little different results:
http://jidesk.net/sypy/strangearc-bryce.png

To draw single and simple "filled" arcs I could use lines and curves, but
that would affect the performances of my script (and would require a lot of
complex calculations), so I'm here to ask if someone can confirm that this
is a bug indeed.

Thanks,
Maurizio

-- 
È difficile avere una convinzione precisa quando si parla delle ragioni del
cuore. - "Sostiene Pereira", Antonio Tabucchi
http://www.jidesk.net

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