ac3: update AC3PreferredStereoDownmixMode.

'11b' is reserved in the A/52 specification,
but newer encoders use it to indicate a Dolby
Pro Logic II compatible Lt/Rt downmix.

libavcodec/ac3.h

@@ -109,7 +109,7 @@ typedef enum AC3PreferredStereoDownmixMode {
     AC3_DMIXMOD_NOTINDICATED = 0,
     AC3_DMIXMOD_LTRT,
     AC3_DMIXMOD_LORO,
-    AC3_DMIXMOD_RESERVED
+    AC3_DMIXMOD_DPLII // reserved value in A/52, but used by encoders to indicate DPL2
 } AC3PreferredStereoDownmixMode;
 
 typedef struct AC3BitAllocParameters {