af_scaletempo2: calculate latency by center of search block

This changes the emitted pts values from the start of the search block
to the center of the search block. Change initial `output_time`
accordingly. Initial `search_block_index` is irrelevant, because it's
overwritten before the first iteration.

Using the `output_time` removes the rounding of `search_block_index`,
which also fixes the <20 microsecond gaps in timestamps between output
packets.

Rationale:

The variance in audio position was in the range `0..search-interval`.

With this change, the range is

    (- search-interval / 2)..(search-interval / 2)`

which ensures lower maximum offset.

1 files changed, 2 insertions, 1 deletions

diff --git a/audio/filter/af_scaletempo2_internals.h b/audio/filter/af_scaletempo2_internals.h
index f1b74ec64f..622d6e20da 100644
--- a/audio/filter/af_scaletempo2_internals.h
+++ b/audio/filter/af_scaletempo2_internals.h
@@ -64,7 +64,8 @@ struct mp_scaletempo2 {
     double output_time;
     // The offset of the center frame of |search_block| w.r.t. its first frame.
     int search_block_center_offset;
-    // Index of the beginning of the |search_block|, in frames.
+    // Index of the beginning of the |search_block|, in frames. This may be
+    // negative, which is handled by |peek_audio_with_zero_prepend|.
     int search_block_index;
     // Number of Blocks to search to find the most similar one to the target
     // frame.