Increase Bezier timing function accuracy

The polyfill evaluation of cubic Bezier timing functions
gives slightly different results than the native Blink
implementation.

In my testing, this change reduced the absolute error by 63%
while only increasing the number of cubic function evaluations
by 31%

This closes #139

src/timing-utilities.js

@@ -168,7 +168,7 @@
         var mid = (start + end) / 2;
         function f(a, b, m) { return 3 * a * (1 - m) * (1 - m) * m + 3 * b * (1 - m) * m * m + m * m * m};
         var xEst = f(a, c, mid);
-        if (Math.abs(x - xEst) < 0.001) {
+        if (Math.abs(x - xEst) < 0.0001) {
           return f(b, d, mid);
         }
         if (xEst < x) {