WaveShaperNode.curve

The current WebKit implementation of this node is racy, since the
processing code only protects against simultaneous setting of the curve
property, not against modifying the contents of the ArrayBuffer.

In the Gecko implementation, I'm just copying the contents of the array
upon setting curve for now, but I think a better fix would be to neuter the
contents of the array, and provide a copy of the original contents of the
array if contents reads the curve property again.

Does this make sense?

Thanks!
--
Ehsan
<http://ehsanakhgari.org/>