Importer functions return a vector of vector of pointers to tracks...

... grouping the channels; rather than one flat vector.

OGG, GStreamer and FFmpeg import were written to allow multiple multi-channel
tracks; others always imported one group of channels.

All of that is reflected in the results returned by the importers, though it
makes no difference yet in AudacityProject::AddImportedTracks (where the
results are used).

src/ondemand/ODDecodeFFmpegTask.cpp

@@ -109,17 +109,17 @@ private:
    int                  mStreamIndex;
 };
 
-auto ODDecodeFFmpegTask::FromList(const std::list<TrackHolders> &channels) -> Streams
+auto ODDecodeFFmpegTask::FromList( const TrackHolders &channels ) -> Streams
 {
    Streams streams;
    streams.reserve(channels.size());
    using namespace std;
    transform(channels.begin(), channels.end(), back_inserter(streams),
-      [](const TrackHolders &holders) {
+      [](const NewChannelGroup &holders) {
          Channels channels;
          channels.reserve(holders.size());
          transform(holders.begin(), holders.end(), back_inserter(channels),
-            mem_fn(&TrackHolders::value_type::get)
+            mem_fn(&NewChannelGroup::value_type::get)
          );
          return channels;
       }

src/ondemand/ODDecodeFFmpegTask.h

@@ -31,7 +31,8 @@ public:
    using Channels = std::vector < WaveTrack* >;
    using Streams = std::vector < Channels >;
 
-   static Streams FromList(const std::list<TrackHolders> &channels);
+   static Streams FromList(
+      const std::vector< std::vector< std::unique_ptr<WaveTrack> > > &channels);
 
    /// Constructs an ODTask
    ODDecodeFFmpegTask(const ScsPtr &scs, Streams &&channels, const std::shared_ptr<FFmpegContext> &context, int streamIndex);