Qt Webkit 2.1 - SVG Issue - Object overlapping

Hi All,
I have written a sample blackboard implementation (using svg and javascript)
to be used on Qt Webkit browser. But it does not working properly with Qt 4.7 and Webkit 2.1
I have tried with Qt 4.6 and Webkit 2.1 but the result is same.

Here is the url of sample blackboard
http://122.200.19.67/blackboard-test/blackboard.html


Bug Reproduction steps:

When you open the sample blackboard, on clicking circle / line / square, you can draw overlap objects on
one another. But when you try doing the same thing using "Auto Draw Circle" button, the javascript injects
objects on svg canvas, the circles drawn do not overlap. There seems to be some issue with overlapping of
objects, or may be transparency of objects.

The same code works properly with
1. Qt 4.6 + native webkit.
2. Qt 4.6 + Webkit 2.0
3. Qt 4.7 + native webkit (i.e. with Webkit 2.0)

It does not work with
1. Qt 4.6 + Webkit 2.1
2. Qt 4.7 + Webkit 2.1

Screenshot

Expected output:
http://122.200.19.67/blackboard-test/working.jpg

Buggy output:
http://122.200.19.67/blackboard-test/not-working.jpg

Is this a bug in Webkit 2.1 or there is some proplem in my blackboard implementation.

I am particularly interested in making this work, as Webkit 2.1 (svg) performs better even if you keep on
drawing / inserting objects. The other versions of webkit becomes slow, and produce jagged edges when you use
freehand tool.

Thanks for helping.

Regards,
Tushar.