GetCurrentPosition() to return the correct value using Pa_GetStreamTime(). Not
only does it not work on every platform, there is no point in setting it during
the callback as GetCurrentPosition() isn't called very often (in comparison)
when updating the play bar. Eventually this should be changed to update the
playbar via a callback to get a position that's a lot more accurate.
Originally committed to SVN as r2798.
918945e03b