output/jack: pass size_t to WriteSamples()
output/jack: WriteSamples() returns size_t
output/jack: pass float* to WriteSamples()
output/jack: cache AudioFormat::channels
output/jack: move jack_ringbuffer_write_space() call to WriteSamples()
output/jack: use jack_ringbuffer_get_write_vector()

Reduce number of libjack calls.