avcodec: blacklist some codecs for multithreading

libavcodec doesn't allow multiple threads on some codecs, so blacklist them
to have only 1. Not sure if it would be easier just to revert few commits to
default whole to 1 thread.

I did't spot any other codecs that would exit if thread_count > 1 in libavcodec.

Fix #3613

modules/codec/avcodec/encoder.c

@@ -499,6 +499,23 @@ int OpenEncoder( vlc_object_t *p_this )
 
         if ( p_sys->i_qmin > 0 && p_sys->i_qmin == p_sys->i_qmax )
             p_context->flags |= CODEC_FLAG_QSCALE;
+        /* These codecs cause libavcodec to exit if thread_count is > 1.
+           See libavcodec/mpegvideo_enc.c:MPV_encode_init
+         */
+        if ( i_codec_id == CODEC_ID_FLV1 ||
+             i_codec_id == CODEC_ID_H261 ||
+             i_codec_id == CODEC_ID_LJPEG ||
+             i_codec_id == CODEC_ID_MJPEG ||
+             i_codec_id == CODEC_ID_H263 ||
+             i_codec_id == CODEC_ID_H263P ||
+             i_codec_id == CODEC_ID_MSMPEG4V1 ||
+             i_codec_id == CODEC_ID_MSMPEG4V2 ||
+             i_codec_id == CODEC_ID_MSMPEG4V3 ||
+             i_codec_id == CODEC_ID_WMV1 ||
+             i_codec_id == CODEC_ID_RV10 ||
+             i_codec_id == CODEC_ID_RV20 ||
+             i_codec_id == CODEC_ID_SVQ3 )
+            p_enc->i_threads = 1;
 
         if ( p_enc->i_threads >= 1 )
             avcodec_thread_init( p_context, p_enc->i_threads );