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.
Merge pull request #1048 from pendragon-andyh/master

WebAudio: Fix for WaveShaper "huge curve" test