require implementations to explicitly support planar LPCM

[jrevels]

Switching between planar and interleaved formats is probably desirable for a wide array of Onda applications.

While there's nothing in the spec that precludes a user from defining and using a planar representation, the spec does not require Onda front-ends to support planar LPCM in the same standardized manner that interleaved support is currently required.

EDIT: this is borderline superseded by #77