ffmpeg: add option stats_period

At present, progress stats are updated at a hardcoded interval of
half a second. For long processes, this can lead to bloated
logs and progress reports.

Users can now set a custom period using option -stats_period
Default is kept at 0.5 seconds.

fftools/ffmpeg.c

@@ -1699,7 +1699,7 @@ static void print_report(int is_last_report, int64_t timer_start, int64_t cur_ti
             last_time = cur_time;
             return;
         }
-        if ((cur_time - last_time) < 500000)
+        if ((cur_time - last_time) < stats_period)
             return;
         last_time = cur_time;
     }