pulsesink: Wait until there is enough room to write an entire segment

pulsesink: Wait until there is enough room to write an entire segment

When trying to write out a segment, wait until there is enough free space
for the entire segment. This helps to reduce ripple in the clock reporting,
where the app might query the playback position while only half a segment
has been written (and is therefore reported by _delay(), even though
the ring buffer has not yet been advanced)