adjust output channels based on AVCodecContext.request_channels in AC3 parser

Originally committed as revision 11412 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/aac_ac3_parser.c

@@ -61,6 +61,13 @@ int ff_aac_ac3_parse(AVCodecParserContext *s1,
                     /* update codec info */
                     avctx->sample_rate = sample_rate;
                     avctx->channels = channels;
+                    /* allow downmixing to mono or stereo for AC3 */
+                    if(avctx->request_channels > 0 &&
+                            avctx->request_channels < channels &&
+                            avctx->request_channels <= 2 &&
+                            avctx->codec_id == CODEC_ID_AC3) {
+                        avctx->channels = avctx->request_channels;
+                    }
                     avctx->bit_rate = bit_rate;
                     avctx->frame_size = samples;
                 }