[cairo] Extents under rotation

[Donn]

Hi,
I have found some email talk about this online, but no solution. When 
you rotate a matrix, draw something and then use path_extents to measure 
it, you get a result that's bigger than it should be.

Here's a demo in Vala:

---

// valac --pkg cairo badextents.vala -o test
// View the img.png file after running "test"

using Cairo;
void main(string[] args) {

   double x1;
   double x2;
   double y1;
   double y2;

   Cairo.ImageSurface surface = new Cairo.ImageSurface
	(Cairo.Format.ARGB32, 500, 500);
   Cairo.Context cr = new Cairo.Context (surface);

   //centre the axes - 0,0 in middle
   var matrix = Cairo.Matrix (1, 0, 0, 1, 250,250 );
   matrix.scale (1, 1);
   cr.transform(matrix);

     cr.arc (0.0, 0.0, 30.0, 0, 2*Math.PI);
     cr.set_source_rgb(1,0,0);
     cr.fill_preserve();

     cr.path_extents (out x1, out y1, out x2, out y2);
     stdout.printf("Extents: %G|%G|%G|%G\n", x1,y1,x2,y2);
     //Draw the bbox as calculated by extents
     cr.rectangle(x1,y1,(x2-x1),(y2-y1));
     cr.set_source_rgb(0,0,0);
     cr.stroke();

     //Now, rotate that circle 45 degrees (and move it over a bit)
     cr.translate(90,0);
     cr.rotate((Math.PI/180.0)*45); //45 degree rot

       cr.arc (0.0, 0.0, 30.0, 0, 2*Math.PI);
       cr.set_source_rgb(1,0,0);
       cr.fill_preserve();

       cr.path_extents (out x1, out y1, out x2, out y2);
       stdout.printf("Extents: %G|%G|%G|%G\n", x1,y1,x2,y2);
       //Draw the bbox as calculated by extents -- this is wrong.
       //Why is the rect so much bigger?
       //Is there any way to get the proper user-space coords?
       cr.rectangle(x1,y1,(x2-x1),(y2-y1));
       cr.set_source_rgb(0,0,0);
       cr.stroke();

   surface.write_to_png ("img.png");
}
---

Is there a way to get the user-space extents of that second circle such 
that the rect fits properly?

In my other code, I am doing an identity_matrix() and then extents. This 
gives me a rect in device space, which is fine. However, I also need 
that rect in user space.

\d