adjustment allowing tests to pass. There was a back and forth with mb…

…utterick in GitHub issue 81 about what value is best. Appears that 1.0e-13 or 1.0e-14 are both good values to use.

BezierKit/Library/Utils.swift

@@ -187,7 +187,8 @@ internal class Utils {
         let q = (2 * a * a * a - 9 * a * b + 27 * c) / 27
         let q2 = q/2
         let discriminant = q2 * q2 + p * p * p / 27
-        if discriminant < -smallValue {
+        let tinyValue = 1.0e-14
+        if discriminant < -tinyValue {
             let r = sqrt(-p * p * p / 27)
             let t = -q / (2 * r)
             let cosphi = t < -1 ? -1 : t > 1 ? 1 : t
@@ -204,7 +205,7 @@ internal class Utils {
             if root3 > root2 {
                 callback(root3)
             }
-        } else if discriminant > smallValue {
+        } else if discriminant > tinyValue {
             let sd = sqrt(discriminant)
             let u1 = crt(-q2 + sd)
             let v1 = crt(q2 + sd)