[cairo] Clipping bug in image surface

[Søren Sandmann]

This program:

     http://www.freedesktop.org/~sandmann/cairobug.c

demonstrates a bug that is present at least in master and 1.12.2. 

The program draws two rectangles, paints them white, and then clips to
those rectangles. Then it draws a green quadrilateral, which is supposed
to be clipped to the white area. By resizing the window you can see that
this is not the case in some situations.

If you set the "const int image" variable to 0, the program will use the
X backend. The bug does not appear in that case.

Soren