libavcodec/vorbisenc: Add missing initial_padding

Vorbis has priming samples at the beginning. If the initial_padding is not
set in the encoder, the total sample count will be one frame fewer than it
should be. The result is that we get a truncated version of encoding.

initial_padding should be set to the frame_size used in
vorbis_encode_frame().

Signed-off-by: Guangyu Sun <gsun@roblox.com>

libavcodec/vorbisenc.c

@@ -1287,6 +1287,7 @@ static av_cold int vorbis_encode_init(AVCodecContext *avctx)
     avctx->extradata_size = ret;
 
     avctx->frame_size = 64;
+    avctx->initial_padding = 1 << (venc->log2_blocksize[1] - 1);
 
     ff_af_queue_init(avctx, &venc->afq);