ffmpeg: update fec patch to match recent ffmpeg

An AVOption has been added to ffmpeg in sept. 2020, making the patch in
contrib redundant. This change is integrated in the contrib ffmpeg
starting from commit 85501986, which updates the used version.

This patch adapts the patches to be applied to ffmpeg and the call to
enable FEC in libopus. Specifically it does the following changes:

- backport the mainline ffmpeg patch that add fec to libopusenc
- add packet_loss reload to adapt its value to network
- modify rules.mak to add patches to the correct platforms
- make media_encoder set the right AVOption to enable fec

Issue: #4857
Change-Id: I614fce2f0cf7fc387973b34323138b07c1767726

contrib/src/ffmpeg/libopusenc-reload-packet-loss-at-encode.patch

+From dba13d03bc8e827fededc20b0ab1f574a1500f2a Mon Sep 17 00:00:00 2001
+From: Philip-Dylan Gleonec <philip-dylan.gleonec@savoirfairelinux.com>
+Date: Thu, 11 Feb 2021 12:25:14 +0100
+Subject: [PATCH 2/2] avcodec/libopusenc: reload packet loss at encode
+
+An estimation of packet loss is required by libopus to compute its FEC
+data. Currently, this estimation is constant, and can not be changed
+after configuration. This means an application using libopus through
+ffmpeg can not adapt the packet loss estimation when the network
+quality degrades.
+
+This patch makes the encoder reload the packet_loss AVOption before
+encoding samples, if fec is enabled. This way an application can modify
+the packet loss estimation by changing the AVOption. Typical use-case
+is a RTP stream, where packet loss can be estimated from RTCP packets.
+
+Signed-off-by: Philip-Dylan Gleonec <philip-dylan.gleonec@savoirfairelinux.com>
+---
+ libavcodec/libopusenc.c | 9 +++++++++
+ 1 file changed, 9 insertions(+)
+
+diff --git a/libavcodec/libopusenc.c b/libavcodec/libopusenc.c
+index 70d17f802b..c18e8ae7fa 100644
+--- a/libavcodec/libopusenc.c
++++ b/libavcodec/libopusenc.c
+@@ -460,6 +460,15 @@ static int libopus_encode(AVCodecContext *avctx, AVPacket *avpkt,
+     int ret;
+     int discard_padding;
+ 
++    if (opus->opts.fec) {
++        ret = opus_multistream_encoder_ctl(opus->enc,
++                                           OPUS_SET_PACKET_LOSS_PERC(opus->opts.packet_loss));
++        if (ret != OPUS_OK)
++            av_log(avctx, AV_LOG_WARNING,
++                   "Unable to set expected packet loss percentage: %s\n",
++                   opus_strerror(ret));
++    }
++
+     if (frame) {
+         ret = ff_af_queue_add(&opus->afq, frame);
+         if (ret < 0)
+-- 
+2.25.1
+

contrib/src/ffmpeg/package.json

@@ -12,7 +12,7 @@
     "patches": [
         "change-RTCP-ratio.patch",
         "rtp_ext_abs_send_time.patch",
-        "libopusenc-enable-FEC.patch",
+        "libopusenc-reload-packet-loss-at-encode.patch",
         "libopusdec-enable-FEC.patch"
     ],
     "win_patches": [

contrib/src/ffmpeg/rules.mak

@@ -363,11 +363,9 @@ 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
-	$(APPLY) $(SRC)/ffmpeg/screen-sharing-x11-fix.patch
-ifdef HAVE_IOS
+	$(APPLY) $(SRC)/ffmpeg/libopusenc-reload-packet-loss-at-encode.patch
 	$(APPLY) $(SRC)/ffmpeg/ios-disable-b-frames.patch
-endif
+	$(APPLY) $(SRC)/ffmpeg/screen-sharing-x11-fix.patch
 	$(UPDATE_AUTOCONFIG)
 	$(MOVE)
 

src/media/media_encoder.cpp

@@ -1058,7 +1058,7 @@ void
 MediaEncoder::initOpus(AVCodecContext* encoderCtx)
 {
     // Enable FEC support by default with 10% packet loss
-    av_opt_set_int(encoderCtx, "enable_fec", fecEnabled_ ? 1 : 0, AV_OPT_SEARCH_CHILDREN);
+    av_opt_set_int(encoderCtx, "fec", fecEnabled_ ? 1 : 0, AV_OPT_SEARCH_CHILDREN);
     av_opt_set_int(encoderCtx, "packet_loss", 10, AV_OPT_SEARCH_CHILDREN);
 }