UNIV::SAMPLERATE matches device SampleRate. Fixes PA with Jack backend

Fixes https://github.com/digego/extempore/issues/160

Tested with portaudio over pulse, as well as JACK with samplerate at 48000 and 44100.
Not entirely sure is there are unintended side-effects for this, but presumably matching
the sample rate of the target even if mismatches didn't crash extempore is reasonable.