This will ensure that we don't leak any memory while playing back
audio.
There is an expectation value in the test that is only set to true when
PulseAudio is present for the moment. When any new implementation is
added for other libraries/platforms, we should hopefully get a CI
failure due to unexpected success in creating the `PlaybackStream`.
To ensure that we clean up our PulseAudio connection whenever audio
output is not needed, add `PulseAudioContext::weak_instance()` to allow
us to check whether an instance exists without creating one.
If we don't clear the callbacks, they may be called after our functions
are deleted.
Disconnecting the stream also doesn't appear to be done automatically
when calling `pa_stream_unref()` for the last time, so let's do that.
We don't want to pull the stream out from under our PulseAudio main
loop, so call these with the lock to ensure that nothing is touching
them.
The `pa_threaded_mainloop_stop()` call does not require lock as it sets
a flag to tell the main loop to exit.
The mutex used to protect from multiple threads creating PulseAudio
contexts simultaneously could remain locked when an application exited.
The static variables' destructors could be called on the main thread
while another thread is running `PulseAudioContext::instance()` and
synchronously connecting to a PulseAudio daemon. This would cause an
assertion in Mutex that it is unlocked upon its destructor being
called.
By creating a static `ScopeGuard` that locks and immediately unlocks
the mutex, we can ensure that the main thread waits for the connection
to succeed or fail. In most cases, this will not take long, but if the
connection is timing out, it could take a matter of seconds.
This adds an abstract `Audio::PlaybackStream` class to allow cross-
platform audio playback to be done in an opaque manner by applications
in both Serenity and Lagom.
Currently, the only supported audio API is PulseAudio, but a Serenity
implementation should be added shortly as well.
