There can never be fractional audio time, and since superclock_t is used to represent audio time, when we
convert from Beat time, we should never, ever return a superclock value that does not correspond to
an integer number of samples.
This fixes a number of bugs, including any use of ARDOUR::Filter which writes a new (audio) file
to disk that must be an integer number of samples long, but may be derived from and later used
by a region that had an audio-time duration that is slightly longer (less than one sample)
than the audio file
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