General possible area of bugs in Cairo is double => integer conversion
when calling the backend interfaces, which take integers; e.g., 
_cairo_gstate_show_surface() has:


        status = _cairo_surface_composite (gstate->operator,
                                           surface,
                                           pattern.source,
                                           gstate->surface,
                                           device_x, device_y,
                                           0, 0,
                                           device_x, device_y,
                                           device_width,
                                           device_height);

It needs something like a

 int device_ix = floor (device_x + 0.5);

and similar. (That function also needs checking that the clipped
path gives consistent results... there are fixed point numbers on
that path, so it's not clear on first glance.)

Possible test case:

 - Take primitives, draw them at integer - epsilon integer + epsilon
   test that the result is pixel-the-same.

Hitting every code path is going to be hard, but getting partial
coverage should already help.