player: get playback time more correctly

This patches allows the player to get playback time more correctly(robustly).
Playback time is `current_pts` now, but I think playback time should be `current_pts - start_pts`.

Example for a tiny sample, https://db.tt/Eg7iJoQm.
The media info is given in https://gist.github.com/henry0312/aa61f690c334a4f57b4f.

Results of `$ mpv --start=1 --pause sample.ts`:

Before:
![sample_ts_before](https://dl.dropboxusercontent.com/u/25391619/sample_ts_before.png)

After:
![sample_ts_after](https://dl.dropboxusercontent.com/u/25391619/sample_ts_after.png)