vorbisdec: read the previous window flag for long windows

When reading sequentially, we are using the actual flag from the previous
frame, but when seeking we do not know what the previous window flag was, so
we need to read it from the bitstream.

libavcodec/vorbisdec.c

@@ -1510,8 +1510,10 @@ static int vorbis_parse_audio_packet(vorbis_context *vc)
     blockflag = vc->modes[mode_number].blockflag;
     blocksize = vc->blocksize[blockflag];
     vlen = blocksize / 2;
-    if (blockflag)
-        skip_bits(gb, 2); // previous_window, next_window
+    if (blockflag) {
+        previous_window = get_bits(gb, 1);
+        skip_bits1(gb); // next_window
+    }
 
     memset(ch_res_ptr,   0, sizeof(float) * vc->audio_channels * vlen); //FIXME can this be removed ?
     memset(ch_floor_ptr, 0, sizeof(float) * vc->audio_channels * vlen); //FIXME can this be removed ?