lavc_conv: do not allow libavcodec to drop subtitles with broken UTF-8

libavcodec normally drops subtitle lines that fail a check for invalid
UTF-8 (their check is slightly broken too, by the way). This was always
annoying and inconvenient, but now there is a mechanism to prevent
it from doing this. Requires newst libavcodec.

sub/lavc_conv.c

@@ -94,6 +94,7 @@ struct lavc_conv *lavc_conv_create(struct mp_log *log, const char *codec_name,
     avctx->time_base = (AVRational) {1, 1000};
 #if LIBAVCODEC_VERSION_MICRO >= 100
     avctx->pkt_timebase = avctx->time_base;
+    avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_IGNORE;
 #endif
     priv->avctx = avctx;
     priv->extradata = talloc_strndup(priv, avctx->subtitle_header,

wscript

@@ -413,7 +413,7 @@ iconv support use --disable-iconv.",
 
 ffmpeg_pkg_config_checks = [
     'libavutil',     '>= 56.8.100',
-    'libavcodec',    '>= 58.10.100',
+    'libavcodec',    '>= 58.16.100',
     'libavformat',   '>= 58.9.100',
     'libswscale',    '>= 5.0.101',
     'libavfilter',   '>= 7.0.101',