Support backwards play, a requirement for scrubbing

Uncomment the line at the top of ControlToolBar::PlayPlayRegion to play
everything backwards and test it

It even works correctly with a time track

src/SampleFormat.cpp

@@ -92,6 +92,24 @@ void ClearSamples(samplePtr src, sampleFormat format,
    memset(src + start*size, 0, len*size);
 }
 
+void ReverseSamples(samplePtr src, sampleFormat format,
+                  int start, int len)
+{
+   int size = SAMPLE_SIZE(format);
+   samplePtr first = src + start * size;
+   samplePtr last = src + (start + len - 1) * size;
+   enum { fixedSize = SAMPLE_SIZE(floatSample) };
+   wxASSERT(size <= fixedSize);
+   char temp[fixedSize];
+   while (first < last) {
+      memcpy(temp, first, size);
+      memcpy(first, last, size);
+      memcpy(last, temp, size);
+      first += size;
+      last -= size;
+   }
+}
+
 void CopySamples(samplePtr src, sampleFormat srcFormat,
                  samplePtr dst, sampleFormat dstFormat,
                  unsigned int len,