new logic for calculating a recording's start time

This is as described in design/time.md. Other aspects of that design
(including using the monotonic clock and adjusting the durations to compensate
for camera clock frequency error) are not implemented yet. No new tests yet.
Just trying to get some flight miles on these ideas as soon as I can.

src/mp4.rs

@@ -1408,7 +1408,7 @@ mod tests {
         let extra_data = input.get_extra_data().unwrap();
         let video_sample_entry_id = db.db.lock().insert_video_sample_entry(
             extra_data.width, extra_data.height, &extra_data.sample_entry).unwrap();
-        let mut output = db.dir.create_writer(&db.syncer_channel, START_TIME, START_TIME, 0,
+        let mut output = db.dir.create_writer(&db.syncer_channel, None, 0,
                                               TEST_CAMERA_ID, video_sample_entry_id).unwrap();
 
         // end_pts is the pts of the end of the most recent frame (start + duration).
@@ -1417,15 +1417,20 @@ mod tests {
         // To write the final packet of this sample .mp4 with a full duration, we need to fake a
         // next packet's pts from the ffmpeg-supplied duration.
         let mut end_pts = None;
+
+        let mut frame_time = START_TIME;
+
         loop {
             let pkt = match input.get_next() {
                 Ok(p) => p,
                 Err(ffmpeg::Error::Eof) => { break; },
                 Err(e) => { panic!("unexpected input error: {}", e); },
             };
-            output.write(pkt.data().expect("packet without data"), pkt.pts().unwrap(),
+            let pts = pkt.pts().unwrap();
+            frame_time += recording::Duration(pkt.duration());
+            output.write(pkt.data().expect("packet without data"), frame_time, pts,
                          pkt.is_key()).unwrap();
-            end_pts = Some(pkt.pts().unwrap() + pkt.duration());
+            end_pts = Some(pts + pkt.duration());
         }
         output.close(end_pts).unwrap();
         db.syncer_channel.flush();