fftools/ffmpeg: fix -t inaccurate recording time

if input start time is not 0 -t is inaccurate doing stream copy,
will record extra duration according to input start time.
it should base on following cases:

input video start time from 60s, duration is 300s,
1. stream copy:
       ffmpeg -ss 40 -t 60 -i in.mp4 -c copy -y out.mp4
   open_input_file() will seek to 100 and set ts_offset to -100,
   process_input() will offset pkt->pts with ts_offset to make it 0,
   so when do_streamcopy() with -t, exits when ist->pts >= recording_time.

2. stream copy with -copyts:
       ffmpeg -ss 40 -t 60 -copyts -i in.mp4 -c copy -y out.mp4
   open_input_file() will seek to 100 and set ts_offset to 0,
   process_input() will keep raw pkt->pts as ts_offset is 0,
   so when do_streamcopy() with -t, exits when
   ist->pts >= (recording_time+f->start_time+f->ctx->start_time).

3. stream copy with -copyts -start_at_zero:
       ffmpeg -ss 40 -t 60 -copyts -start_at_zero -i in.mp4 -c copy -y out.mp4
   open_input_file() will seek to 120 and set ts_offset to -60 as start_to_zero option,
   process_input() will offset pkt->pts with input file start time,
   so when do_streamcopy() with -t, exits when ist->pts >= (recording_time+f->start_time).

0      60     40      60                     360
|_______|_____|_______|_______________________|
      start   -ss     -t

This fixes ticket #9141.

Signed-off-by: Shiwang.Xie <shiwang.xie666@outlook.com>

fftools/ffmpeg.c

@@ -2088,9 +2088,11 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
     }
 
     if (f->recording_time != INT64_MAX) {
-        start_time = f->ctx->start_time;
-        if (f->start_time != AV_NOPTS_VALUE && copy_ts)
-            start_time += f->start_time;
+        start_time = 0;
+        if (copy_ts) {
+            start_time += f->start_time != AV_NOPTS_VALUE ? f->start_time : 0;
+            start_time += start_at_zero ? 0 : f->ctx->start_time;
+        }
         if (ist->pts >= f->recording_time + start_time) {
             close_output_stream(ost);
             return;