libtriple: improve cPlayback()

* avoid race conditions by using a mutex to protect curr_pos, fixing
  artefacts during timeshift playback
* make GetPosition() cheaper by interpolating end_pts and only fetching
  the "real" end_pts after the file has grown by 10MB

lib/libtriple/playback_td.h

@@ -77,6 +77,7 @@ class cPlayback
 
 		int64_t pts_start;
 		int64_t pts_end;
+		int64_t _pts_end; /* last good endpts */
 		int64_t pts_curr;
 		int64_t get_pts(uint8_t *p, bool pes, int bufsize);