Playback position with AudioBufferSourceNode

Hi,

We need to get the current playback position of the AudioBufferSourceNode
but we can't seem to find anything about it. At the moment we can calculate
it by subtracting the time it starting playing from the
context.currentTime, but this assumes playback speed is constant -- which
soon it won't be. Has it been implemented but not documented? I hope so,
because the only other option would be the find the integral of the
playbackRate changes since it started playing...

Cheers,

Rob Clouth