avcodec: disable direct rendering for VP80

avcodec native decoder has priority over libvpx one, and I don't know
howto change that easily. Fix #3820

modules/codec/avcodec/video.c

@@ -306,6 +306,9 @@ int InitVideoDec( decoder_t *p_dec, AVCodecContext *p_context,
        (p_sys->p_codec->capabilities & CODEC_CAP_DR1) &&
         /* No idea why ... but this fixes flickering on some TSCC streams */
         p_sys->i_codec_id != CODEC_ID_TSCC &&
+        /* avcodec native vp8 decode doesn't handle EMU_EDGE flag, and I
+           don't have idea howto implement fallback to libvpx decoder */
+        p_sys->i_codec_id != CODEC_ID_VP8 &&
         !p_sys->p_context->debug_mv )
     {
         /* Some codecs set pix_fmt only after the 1st frame has been decoded,