Fixes to allow changing stream format

[plietar]

[mitchmindtree]

Hey @plietar thanks for the PR :)

Can you explain a little more the reasoning behind changing the Scope of the audio unit property request from Output to Input? Is this a bug fix? How exactly does the behaviour change?

[plietar]

Sure, disclaimer though, I've got very little experience with CoreAudio. My understanding is that each unit has an input and an output format. For a sink only the input format matters.

Before this patch, setting the format worked (set_stream_format changes the input), but setting a render callback would not, since it was checking against the output (https://github.com/RustAudio/coreaudio-rs/blob/master/src/audio_unit/render_callback.rs#L387-L394)

This PR makes stream_format and set_stream_format consistent. If this library ever supports audio sources, then this should be revisited (ie make these two methods take a scope, or make two more methods)

[akhudek]

I've recently been working on using this crate for audio input and encountered the scope problem, which is actually a bit bigger than just this change. It took a lot of searching but Figure 1-3 in https://developer.apple.com/library/archive/documentation/MusicAudio/Conceptual/AudioUnitHostingGuide_iOS/AudioUnitHostingFundamentals/AudioUnitHostingFundamentals.html makes it clear.

For rendering audio, your application should use the input scope and element 0 (aka element::Output). For taking audio from the microphone you need to use the output scope and element 1 (aka element::Input).