Downgrade cpal to PR/feature branch adding OpenBSD support

Done with editing Cargo.toml followed by `cargo update -p cpal`,
which, sadly, change a whole bunch of other dependencies.

This makes audio play, but two issues remain:

1. Playback speed is noticably slow.
2. Toggling play -> shows this error, although playback is successfully
   paused:
   ```
   [2021-11-17T16:03:04Z ERROR psst_core::audio::output] failed to stop stream: A backend-specific error has occurred: pausing is not implemented
   ```
"Add support for OpenBSD platform via sndio host"
RustAudio/cpal#493 is based on an older cpal
release;  maybe rebasing on top of HEAD and therefore bringing in the
cpal changes that current psst already uses helps with/fixes any of the
two issues?

Cargo.toml

@@ -15,3 +15,5 @@ opt-level = 2
 piet = { git = "https://github.com/jpochyla/piet", branch = "ctfont-cache" }
 piet-common = { git = "https://github.com/jpochyla/piet", branch = "ctfont-cache" }
 piet-coregraphics = { git = "https://github.com/jpochyla/piet", branch = "ctfont-cache" }
+# (not merged yet) solve panic at startup: "Add support for OpenBSD platform via sndio host" https://github.com/RustAudio/cpal/pull/493
+cpal = { git = "https://github.com/conwayste/cpal", branch = "aaron/openbsd_sndio_host" }