swr/auto_matrix: treat stereo downmix like stereo.

Based on code by John Stebbins
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/rematrix.c

@@ -103,6 +103,8 @@ static int even(int64_t layout){
 }
 
 static int clean_layout(SwrContext *s, int64_t layout){
+    if((layout & AV_CH_LAYOUT_STEREO_DOWNMIX) == AV_CH_LAYOUT_STEREO_DOWNMIX)
+        return AV_CH_LAYOUT_STEREO;
 
     return layout;
 }