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).
Make it possible to specify different channels and positions
per stream so that we can do remapping as well.
Make sure we only copy available input channels and set the remaining
channels empty.
Handle error when loading and fix leaks.
Mark streams as virtual if nothing else is specified.
Try to reuse an existing core connection if possible.
unload the module when the core is in error or when it is destroyed.
