Warn if Vorbis does not support the provided channel layout.

The warning is printed with loglevel error because the output
stream is known to be broken.
Also warn if no channel layout was provided for multichannel input.

libavcodec/libvorbis.c

@@ -96,6 +96,35 @@ static av_cold int oggvorbis_init_encoder(vorbis_info *vi, AVCodecContext *avcco
         vorbis_encode_ctl(vi, OV_ECTL_IBLOCK_SET, &context->iblock);
     }
 
+    if (avccontext->channels == 3 &&
+            avccontext->channel_layout != (AV_CH_LAYOUT_STEREO|AV_CH_FRONT_CENTER) ||
+        avccontext->channels == 4 &&
+            avccontext->channel_layout != AV_CH_LAYOUT_2_2 &&
+            avccontext->channel_layout != AV_CH_LAYOUT_QUAD ||
+        avccontext->channels == 5 &&
+            avccontext->channel_layout != AV_CH_LAYOUT_5POINT0 &&
+            avccontext->channel_layout != AV_CH_LAYOUT_5POINT0_BACK ||
+        avccontext->channels == 6 &&
+            avccontext->channel_layout != AV_CH_LAYOUT_5POINT1 &&
+            avccontext->channel_layout != AV_CH_LAYOUT_5POINT1_BACK ||
+        avccontext->channels == 7 &&
+            avccontext->channel_layout != (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_CENTER) ||
+        avccontext->channels == 8 &&
+            avccontext->channel_layout != AV_CH_LAYOUT_7POINT1) {
+        if (avccontext->channel_layout) {
+            char name[32];
+            av_get_channel_layout_string(name, sizeof(name), avccontext->channels,
+                                         avccontext->channel_layout);
+            av_log(avccontext, AV_LOG_ERROR, "%s not supported by Vorbis: "
+                                             "output stream will have incorrect "
+                                             "channel layout.\n", name);
+        } else {
+            av_log(avccontext, AV_LOG_WARNING, "No channel layout specified. The encoder "
+                                               "will use Vorbis channel layout for "
+                                               "%d channels.\n", avccontext->channels);
+        }
+    }
+
     return vorbis_encode_setup_init(vi);
 }