fix WaveTrackCache, again...

... There was still some duplicated loading of block files during normal
forward play, though not more than two loads per file.

src/WaveTrack.cpp

@@ -2661,9 +2661,7 @@ constSamplePtr WaveTrackCache::Get(sampleFormat format,
          // Request starts in the second buffer and extends past it.
          // Discard the first buffer.
          // (But don't deallocate the buffer space.)
-         float *save = mBuffers[0].data;
-         mBuffers[0] = mBuffers[1];
-         mBuffers[1].data = save;
+         mBuffers[0] .swap ( mBuffers[1] );
          fillSecond = true;
          mNValidBuffers = 1;
       }
@@ -2678,9 +2676,7 @@ constSamplePtr WaveTrackCache::Get(sampleFormat format,
          // refill the first.
          // (This case might be useful when marching backwards through
          // the track, as with scrubbing.)
-         float *save = mBuffers[1].data;
-         mBuffers[1] = mBuffers[0];
-         mBuffers[0].data = save;
+         mBuffers[0] .swap ( mBuffers[1] );
          fillFirst = true;
          fillSecond = false;
          // Cache is not in a consistent state yet

src/WaveTrack.h

@@ -646,6 +646,13 @@ private:
       Buffer() : data(0), start(0), len(0) {}
       void Free() { delete[] data; data = 0; start = 0; len = 0; }
       sampleCount end() const { return start + len; }
+
+      void swap ( Buffer &other )
+      {
+         std::swap( data, other.data );
+         std::swap( start, other.start );
+         std::swap( len, other.len );
+      }
    };
 
    const WaveTrack *mPTrack;