[osx] support planar audio devices in core audio OSX sink

Some fancy-schmancy OSX audio devices support planar audio by specifying that they have a bunch of streams that are single-channels.  The higher level API handles this fine, but we don't as we're using the low-level API.  Fortunately these cases are pretty simple to detect.

As AE currently doesn't support planar audio to the sink, we de-interleave in the output.  Once AE supports it, much of the changes in the render callback can be removed.

Gotham material.