[cairo] Backend API composite_trapezoids question

[Per Persson]

Hi, 
I've been playing with creating a "toy" backend for a few days and I'm a little confused about the data I receive in the composite_trapezoids() case.
Now, this is most likely a misunderstanding on my part regarding what to expect. I'll show an example of what I mean.

Consider the following cairo snippet

  cairo_set_line_width(cr, 20.0);
  cairo_set_source_rgba (cr, 0.5, 0.5, 0.3, 0.5);
  cairo_move_to (cr, 100.0, 50.0);
  cairo_line_to(cr, 200.0, 250.0);
  cairo_line_to(cr, 150.0, 300.0);
  cairo_line_to(cr, 50.0, 150.0);
  cairo_close_path(cr);
  cairo_stroke(cr);

As far as I understand things, the tessellated path is passed to composite_trapezoids() where I intercept it.
In composite_trapezoids() I naively converted the trapezoids to polygons, but I find that some of the polygons overlap which means that when alpha is lee than one, I get artefacts in the "inner corners" of the rendered path. 

See 
http://homepage.mac.com/persquare/cairo/artefacts.png
for an example of the artefacts, and
http://homepage.mac.com/persquare/cairo/polygons.pdf
for a picture of the polygons derived from the original trapezoids.

Is this behaviour is expected? 
If so, some post-processing of the trapezoids is obviuosly required and I'd be happy if anyone could provide me with pointers to some reasoning how 
this is supposed to work.

Best regards,
Per