ffmpeg: add FEC support to libopusenc

Add option to enable FEC support in libopus encoder, and adds uses this
option to instanciate the opus encoder.
This option is disabled by default, in order to avoid changing default
ffmpeg behavior. If FEC support is enabled and packet loss is higher
than 0, the encoder adds ancilliary data to the packet so the decoder
can restore the previous packet, in case it is lost. As more data has
to be transmitted, this reduces the audio quality when using a fixed
bitrate.

Issue: #5156
Change-Id: Ic7d69ab122bd63c3ff880e46a0a0dcb316b00760

contrib/src/ffmpeg/libopusenc-enable-FEC.patch

+diff --git a/libavcodec/libopusencc.c b/libavcodec/libopusencc.c
+index 2a97811d18..40ee7b8fec 100644
+--- a/libavcodec/libopusenc.c
++++ b/libavcodec/libopusenc.c
+@@ -33,6 +33,7 @@
+ typedef struct LibopusEncOpts {
+     int vbr;
+     int application;
++    int enable_fec;
+     int packet_loss;
+     int complexity;
+     float frame_duration;
+@@ -143,6 +144,13 @@
+                "Unable to set constrained VBR: %s\n", opus_strerror(ret));
+
+     ret = opus_multistream_encoder_ctl(enc,
++                                       OPUS_SET_INBAND_FEC(opts->enable_fec));
++    if (ret != OPUS_OK)
++        av_log(avctx, AV_LOG_WARNING,
++               "Unable to set enable FEC flag percentage: %s\n",
++               opus_strerror(ret));
++
++    ret = opus_multistream_encoder_ctl(enc,
+                                        OPUS_SET_PACKET_LOSS_PERC(opts->packet_loss));
+     if (ret != OPUS_OK)
+         av_log(avctx, AV_LOG_WARNING,
+@@ -543,6 +551,7 @@
+         { "audio",          "Favor faithfulness to the input",         0, AV_OPT_TYPE_CONST, { .i64 = OPUS_APPLICATION_AUDIO },               0, 0, FLAGS, "application" },
+         { "lowdelay",       "Restrict to only the lowest delay modes", 0, AV_OPT_TYPE_CONST, { .i64 = OPUS_APPLICATION_RESTRICTED_LOWDELAY }, 0, 0, FLAGS, "application" },
+     { "frame_duration", "Duration of a frame in milliseconds", OFFSET(frame_duration), AV_OPT_TYPE_FLOAT, { .dbl = 20.0 }, 2.5, 120.0, FLAGS },
++    { "enable_fec",     "Enable forward error correction",     OFFSET(enable_fec),     AV_OPT_TYPE_BOOL,  { .i64 = 0 },    0,   1,     FLAGS },
+     { "packet_loss",    "Expected packet loss percentage",     OFFSET(packet_loss),    AV_OPT_TYPE_INT,   { .i64 = 0 },    0,   100,  FLAGS },
+     { "vbr",            "Variable bit rate mode",              OFFSET(vbr),            AV_OPT_TYPE_INT,   { .i64 = 1 },    0,   2,    FLAGS, "vbr" },
+         { "off",            "Use constant bit rate", 0, AV_OPT_TYPE_CONST, { .i64 = 0 }, 0, 0, FLAGS, "vbr" },

contrib/src/ffmpeg/rules.mak

@@ -352,6 +352,7 @@ ffmpeg: ffmpeg-$(FFMPEG_HASH).tar.gz
 	$(APPLY) $(SRC)/ffmpeg/change-RTCP-ratio.patch
 	$(APPLY) $(SRC)/ffmpeg/rtp_ext_abs_send_time.patch
 	$(APPLY) $(SRC)/ffmpeg/libopusdec-enable-FEC.patch
+	$(APPLY) $(SRC)/ffmpeg/libopusenc-enable-FEC.patch
 	$(UPDATE_AUTOCONFIG)
 	$(MOVE)