Fix inappropriate approximation of Math.exp

FastMath.exp was neither continuous nor strictly monotonically increasing for x < -1 and therefore inappropriate for the intended purpose.

brouter-util/src/main/java/btools/util/FastMath.java

@@ -7,15 +7,13 @@
  */
 public class FastMath {
   /**
-   * Approximation to Math.exp for small negative arguments
+   * Approximation to Math.exp
    */
   public static double exp(double e) {
-    double x = e;
-    double f = 1.;
-    while (e < -1.) {
-      e += 1.;
-      f *= 0.367879;
-    }
-    return f * (1. + x * (1. + x * (0.5 + x * (0.166667 + 0.0416667 * x))));
+    e = 1 + e / 64;
+    e *= e; e *= e;
+    e *= e; e *= e;
+    e *= e; e *= e;
+    return e;
   }
 }