spumux, some captions show a bug

I'm just trying to render some subtitles using spumux (latest git, vanilla `untouched' snapshot) and some of them show a bad rendering in some circumstances.

Here you can see them: http://imageshack.us/photo/my-images/13/comparisonso.png/
(the first one and the last one are rendered the right way)
As you can see, the second and the third captions show a cut, in other words, an incomplete rendering affecting the bottom pixels. It seems some kind of buffer was prepared to host the rendered image but that buffer was too little and some part of the image was not rendered.
In detail, it seems also the text has a role: in sentences with such a character as `p', `q', `g' or other ones with a `bottom propagation' the image is fully rendered. Even a comma may do the trick (latest caption) and help to achieve a perfect rendering.
Now, I really can't see where the bug can hide, I'm figuring out the rendering engine is involved but I don't know that part of the code.
I can provide more details if needed, any help is appreciated.
Thank you,
Elio


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