In section '6.13.5 Calculating the current iteration', Step 4 should check for the case where we are at the end of an integer number of iterations. In this case, we should prevent current iteration from 'overflowing' to the integer value. This is similar to the check used when calculating unscaled iteration time (see Bug 20094).

This fix is required for fill to work correctly with iterations.

The corresponding fix for the polyfill is https://github.com/steveblock/web-anim-js/commit/b510208fbdc63fa598a2268b63e3c0b8cfe64c65