lavc/libgsmenc: Force mono and use 13k as default bitrate.

libavcodec/libgsmenc.c

@@ -114,6 +114,10 @@ static int libgsm_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     return 0;
 }
 
+static const AVCodecDefault libgsm_defaults[] = {
+    { "b",                "13000" },
+    { NULL },
+};
 
 #if CONFIG_LIBGSM_ENCODER
 AVCodec ff_libgsm_encoder = {
@@ -124,6 +128,8 @@ AVCodec ff_libgsm_encoder = {
     .init           = libgsm_encode_init,
     .encode2        = libgsm_encode_frame,
     .close          = libgsm_encode_close,
+    .defaults       = libgsm_defaults,
+    .channel_layouts= (const uint64_t[]) { AV_CH_LAYOUT_MONO, 0 },
     .sample_fmts    = (const enum AVSampleFormat[]){ AV_SAMPLE_FMT_S16,
                                                      AV_SAMPLE_FMT_NONE },
     .wrapper_name   = "libgsm",
@@ -138,6 +144,8 @@ AVCodec ff_libgsm_ms_encoder = {
     .init           = libgsm_encode_init,
     .encode2        = libgsm_encode_frame,
     .close          = libgsm_encode_close,
+    .defaults       = libgsm_defaults,
+    .channel_layouts= (const uint64_t[]) { AV_CH_LAYOUT_MONO, 0 },
     .sample_fmts    = (const enum AVSampleFormat[]){ AV_SAMPLE_FMT_S16,
                                                      AV_SAMPLE_FMT_NONE },
     .wrapper_name   = "libgsm",