Fix CPAL buffer bug (standalone)

[JerwuQu]

This is trying to solve the same thing as #104.

nih-plug master gives distorted audio for me when using wasapi or alsa backends when built standalone.

104 fixes this, but for me it only worked for one channel, while this worked for all channels.

Tested working on Windows/WASAPI and Linux/Alsa. Further testing would be good too.

[robbert-vdh]

Thanks for the PR! There were two (or well, three) issues here:

• The current implementation reads but doesn't actually write in an interleaved fashion. This PR fixes that for the writing part.
• CPAL promises us that we'll receive a certain number of samples per buffer, but it gives us different amounts for the audio callback. Using data.len() / num_output_channels instead of the configured buffer size works as long as that number is below the configured buffer size (i.e. as long as these constraints still holds true). This is a bit annoying and there are also situations where you get more samples than you requested (e.g. https://github.com/RustAudio/cpal/issues/748), but I guess this behavior is still better than what it is now.