SPA_MEMBER is misleading, all we're doing here is pointer+offset and a
type-casting the result. Rename to SPA_PTROFF which is more expressive (and
has the same number of characters so we don't need to re-indent).
Only implement the null echo cancel implementation for now.
And skeleton webrtc echo cancel implementation
It uses 4 streams arranged as:
input ---+---> source
^
|
sink ---+---> output
The output of the source is the filtered input of the input stream
(linked a master source) based on the data going from sink to
the output (linked to a master sink).
All streams are arranged in the same group so that the echo canceler
does not have to deal with clock drift. For echo cancelers that can
handle clock drift we might want place the source and sink chains
in different groups.
