Optimize AudioBuffer copying

[ferjm]

Apply the optimizations suggested by the spec here:

When an acquire the content operation is performed on an AudioBuffer, the entire operation can usually be implemented without copying channel data. In particular, the last step SHOULD be performed lazily at the next getChannelData() call. That means a sequence of consecutive acquire the contents operations with no intervening getChannelData() (e.g. multiple AudioBufferSourceNodes playing the same AudioBuffer) can be implemented with no allocations or copying.
Implementations can perform an additional optimization: if getChannelData() is called on an AudioBuffer, fresh ArrayBuffers have not yet been allocated, but all invokers of previous acquire the content operations on an AudioBuffer have stopped using the AudioBuffer's data, the raw data buffers can be recycled for use with new AudioBuffers, avoiding any reallocation or copying of the channel data.