No sound on Ubuntu 18.04 with Pulseaudio

[teknico]

When running cargo run on Ubuntu 18.04 with Pulseaudio with under v.1.30.1, an unsupported format is chosen by cpal and the "Can't handle output buffer format" panic results.

Once the one format supported by the code is hard-wired, stream_data yields a buffer that is not a multiple of 2 * N_SAMPLES_PER_CHUNK, causing an "index out of bounds" panic.