Add two new gradient columns, one where the start circle is has radius
0 and one where the end circle has radius 0. All the new gradients
except for one are rendered with a bright dot in the middle. In most
but not all cases this is incorrect.

Cc: ranma42@gmail.com

This test runs the new floating point combiners on random input with
divide-by-zero exceptions turned on.

With the floating point combiners the only thing we guarantee is that
divide-by-zero exceptions are not generated, so change
enable_fp_exceptions() to only enable those, and rename accordingly.

This separates the test suite from the random gtk+ using test
programs. "demos" is somewhat misleading because the programs there
are not particularly exciting (with the possible exception of
composite-test which shows off all the compositing operators).

radial-test is a port of the radial-gradient test from the cairo test
suite. It has been modified so that some pixels have 0 in both the a
and b coefficients of the quadratic equation solved by the rasterizer,
to expose a division by zero in the original implementation.