[cairo] Extension of the radial gradient test

[Emmanuel Pacaud]

Hi,

Here's a patch that extends the current radial gradient tests with some
corner cases.

Additionnal tests are:

(a) a test with r0 > r1 ;
(b) a test with r0 == 0 and located on the outer circle edge ;
(c) a test where r1 / (r1 - r0) is not an integer, that may show issue
in rednering the content of the inner circle ;
(d) a test with r1 == r0.

The result with the image backend is here:

http://emmanuel.pacaud.free.fr/screenshots/cairo/radial-gradient-image-argb32-out.png

It looks like the image backend fails for (a) in the CAIRO_EXTEND_NONE
and CAIRO_EXTEND_PAD cases, and for (b) in all cases.

And the result with the SVG backend, rendered using batik is here:

http://emmanuel.pacaud.free.fr/screenshots/cairo/radial-gradient-svg-batik.png

It shows a failure in (c) for CAIRO_EXTEND_REFLECT.

Regarding the (d) test, for all extends except CAIRO_EXTEND_NONE, the
result is erased, since I'm not sure what to expect. The main goal is to
detect crashes in backend implementation.

During my recent work on radial gradient for the SVG backend, I tried to
take care of these corner cases. But I think (a) and (d) should be
handled by cairo before calling the backends, in order to avoid bugs in
each backend implementation.

	Emmanuel.


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