avcodec/frame_thread_encoder: warn about huffyuv limitations

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/frame_thread_encoder.c

@@ -144,6 +144,8 @@ int ff_frame_thread_encoder_init(AVCodecContext *avctx, AVDictionary *options){
         avctx->codec_id == AV_CODEC_ID_FFVHUFF) {
         // huffyuv doesnt support these with multiple frame threads currently
         if (avctx->context_model > 0 || (avctx->flags & CODEC_FLAG_PASS1)) {
+            av_log(avctx, AV_LOG_WARNING,
+               "Forcing thread count to 1 for huffyuv encoding with first pass or context 1\n");
             avctx->thread_count = 1;
         }
     }