[Pixman] Some test suite improvements

[Søren Sandmann]

Here is a set of patches that contain some updates to the test
suite. Specifically,

- gradient-crash-test is extended to test more scenarios

- floating point exceptions are enabled in some cases

- The argument to fence_malloc() becomes a signed integer, and it will
  abort() if someone tries to malloc a negative size.

- A new stress-test program that tries to use various combinations of
  unusual features. The hope is that this will provoke crashes or
  irregular behavior.


Soren