ffmpeg: add option -isync

This is a per-file input option that adjusts an input's timestamps
with reference to another input, so that emitted packet timestamps
account for the difference between the start times of the two inputs.

Typical use case is to sync two or more live inputs such as from capture
devices. Both the target and reference input source timestamps should be
based on the same clock source.

If either input lacks starting timestamps, then no sync adjustment is made.

fftools/ffmpeg.h

@@ -118,6 +118,7 @@ typedef struct OptionsContext {
     float readrate;
     int accurate_seek;
     int thread_queue_size;
+    int input_sync_ref;
 
     SpecifierOpt *ts_scale;
     int        nb_ts_scale;
@@ -410,6 +411,7 @@ typedef struct InputFile {
                              at the moment when looping happens */
     AVRational time_base; /* time base of the duration */
     int64_t input_ts_offset;
+    int input_sync_ref;
 
     int64_t ts_offset;
     int64_t last_ts;