Frac() should always produce results in the range [0.0, 1.0), and thus
never produce a 1.0. However, the current implementation uses
x - floor(x) and this returns 1.0 for very small negative values due to
catastrophic cancellation.

Bug swiftshader:74

Change-Id: I942dd7cfb1f7ee3a260070e748704f005eed0b13
Reviewed-on: https://swiftshader-review.googlesource.com/10648Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>