WebAudio: Fix for WaveShaper "huge curve" test

The test previously had an even number of elements so the centre of the
curve was actually mid-way between 2 elements (so I was using the wrong
value used as the centre).  I have now changed the curve to be an odd
number of elements.

Firefox nightly now has 100% passes.

This Pull Request replaces #1041.  @chrislo requested that I spin-up a new branch and pull just a single commit.