In commit 6628042f, audio upload was
moved from retro_run() to S9xDeinitUpdate(). This breaks audio when
runahead is enabled in RetroArch.

With second-instance runahead, S9xDeinitUpdate() is not called when
video rendering is disabled and thus the core instance responsible for
audio is not uploading the audio. With single-instance runahead, audio
is uploaded twice because video rendering is always enabled and thus
S9xDeinitUpdate() gets called twice per frame.

Fix this by introducing a callback that gets called at the end of every
screen refresh, regardless of whether or not rendering is active for
this frame. We can then decide in the callback whether or not audio
should be uploaded.