transcode: rewrite fps/lipsync code

Add some comments on how the code should work. Basic idea is first
check input lipsync against audio track. Then check if we can drop the
picture based on output pts and then do same checked in OutputFrame as
some filters can output multiple pictures (yadif2x etc).

simple framerate conversion from 25 worked to 4,5,10,20,23,29.97,50.60.
Not sure if yadif2x and all work fully yet.

modules/stream_out/transcode/audio.c

@@ -204,7 +204,7 @@ int transcode_audio_process( sout_stream_t *p_stream,
     {
         if( p_sys->b_master_sync )
         {
-            mtime_t i_pts = date_Get( &id->interpolated_pts ) + 1;
+            mtime_t i_pts = date_Get( &id->interpolated_pts );
             mtime_t i_drift = 0;
 
             if( likely( p_audio_buf->i_pts != VLC_TS_INVALID ) )
@@ -217,7 +217,7 @@ int transcode_audio_process( sout_stream_t *p_stream,
                     "audio drift is too high (%"PRId64"), resetting master sync",
                     i_drift );
                 date_Set( &id->interpolated_pts, p_audio_buf->i_pts );
-                i_pts = p_audio_buf->i_pts + 1;
+                i_pts = date_Get( &id->interpolated_pts );
             }
             if( likely(p_audio_buf->i_pts != VLC_TS_INVALID ) )
                 p_sys->i_master_drift = p_audio_buf->i_pts - i_pts;

modules/stream_out/transcode/transcode.h

@@ -11,6 +11,7 @@
 
 #include <vlc_picture_fifo.h>
 
+/*100ms is around the limit where people are noticing lipsync issues*/
 #define MASTER_SYNC_MAX_DRIFT 100000
 
 struct sout_stream_sys_t
@@ -99,7 +100,10 @@ struct sout_stream_id_t
     encoder_t       *p_encoder;
 
     /* Sync */
-    date_t          interpolated_pts;
+    date_t          interpolated_pts; /**< Incoming calculated PTS */
+    date_t          next_output_pts; /**< output calculated PTS */
+    int             i_output_frame_interval;
+    int             i_input_frame_interval;
 };
 
 /* OSD */