swr: generate more dither noise to improve quality.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/swresample_internal.h

@@ -50,6 +50,7 @@ struct SwrContext {
     const int *channel_map;                         ///< channel index (or -1 if muted channel) map
     int used_ch_count;                              ///< number of used input channels (mapped channel count if channel_map, otherwise in.ch_count)
     enum SwrDitherType dither_method;
+    int dither_pos;
 
     int int_bps;                                    ///< internal bytes per sample
     int resample_first;                             ///< 1 if resampling must come first, 0 if rematrixing