atan(0, <negative>) was returning 0 instead of PI. Did a simple
fix in the arctan() function and added an associated unit test.

Change-Id: Idbbdaf099b5104e3aaa2868ca8fd806c6c735981
Reviewed-on: https://swiftshader-review.googlesource.com/18868Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>