Support 32 bit sources in downmix

[ftab]

Checklist

• Checked the issue tracker for similar issues to ensure this is not a duplicate.
• Described the feature in detail and justified the reason for the request.
• Provided specific use cases and examples.

Feature description

It looks as if downmix is hardcoded to 16 bit only.

This should be fixed so that downmix can be used with 24 or 32 bit sources.

Use cases

Playing a 32 bit primary source (I2S), and mixing in a 16 bit secondary source (mp3 files for sound effects)

Alternatives

No response

Additional context

No response