AVcodec: disable slice_thread + HW decoding in MPEG-2

Close #5867
(cherry picked from commit e3bb4aa9)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/avcodec/video.c

@@ -355,7 +355,10 @@ int InitVideoDec( decoder_t *p_dec, AVCodecContext *p_context,
          i_codec_id == CODEC_ID_VC1 || i_codec_id == CODEC_ID_WMV3) )
     {
 #ifdef HAVE_AVCODEC_MT
-        if( p_sys->p_context->thread_type & FF_THREAD_FRAME )
+        if( ( p_sys->p_context->thread_type & FF_THREAD_FRAME ) ||
+            ( ( p_sys->p_context->thread_type & FF_THREAD_SLICE ) &&
+              ( i_codec_id == CODEC_ID_MPEG1VIDEO || i_codec_id == CODEC_ID_MPEG2VIDEO ) )
+          )
         {
             msg_Warn( p_dec, "threaded frame decoding is not compatible with ffmpeg-hw, disabled" );
             p_sys->p_context->thread_type &= ~FF_THREAD_FRAME;