ffplay: honor SDL_AUDIO_CHANNELS and make sure to use SDL supported number of audio channels

Fixes ticket #838.
Signed-off-by: Marton Balint <cus@passwd.hu>

ffplay.c

@@ -2221,6 +2221,8 @@ static int stream_component_open(VideoState *is, int stream_index)
     AVDictionary *opts;
     AVDictionaryEntry *t = NULL;
     int64_t wanted_channel_layout = 0;
+    int wanted_nb_channels;
+    const char *env;
 
     if (stream_index < 0 || stream_index >= ic->nb_streams)
         return -1;
@@ -2257,8 +2259,19 @@ static int stream_component_open(VideoState *is, int stream_index)
         avctx->flags |= CODEC_FLAG_EMU_EDGE;
 
     if (avctx->codec_type == AVMEDIA_TYPE_AUDIO) {
-        wanted_channel_layout = (avctx->channel_layout && avctx->channels == av_get_channel_layout_nb_channels(avctx->channels)) ? avctx->channel_layout : av_get_default_channel_layout(avctx->channels);
-        wanted_channel_layout &= ~AV_CH_LAYOUT_STEREO_DOWNMIX;
+        env = SDL_getenv("SDL_AUDIO_CHANNELS");
+        if (env)
+            wanted_channel_layout = av_get_default_channel_layout(SDL_atoi(env));
+        if (!wanted_channel_layout) {
+            wanted_channel_layout = (avctx->channel_layout && avctx->channels == av_get_channel_layout_nb_channels(avctx->channels)) ? avctx->channel_layout : av_get_default_channel_layout(avctx->channels);
+            wanted_channel_layout &= ~AV_CH_LAYOUT_STEREO_DOWNMIX;
+            wanted_nb_channels = av_get_channel_layout_nb_channels(wanted_channel_layout);
+            /* SDL only supports 1, 2, 4 or 6 channels at the moment, so we have to make sure not to request anything else. */
+            while (wanted_nb_channels > 0 && (wanted_nb_channels == 3 || wanted_nb_channels == 5 || wanted_nb_channels > 6)) {
+                wanted_nb_channels--;
+                wanted_channel_layout = av_get_default_channel_layout(wanted_nb_channels);
+            }
+        }
         wanted_spec.channels = av_get_channel_layout_nb_channels(wanted_channel_layout);
         wanted_spec.freq = avctx->sample_rate;
         if (wanted_spec.freq <= 0 || wanted_spec.channels <= 0) {