Fix test_pipe()

When trimming, FFmpeg 5.x retains the input presentation timestamp (PTS)
in the output.  This causes the trimmed output frames to be duplicates
of the first.  To prevent this, the output PTS needs to be recalculated.

ffmpeg/tests/test_ffmpeg.py

@@ -688,8 +688,8 @@ def test_pipe():
     width = 32
     height = 32
     frame_size = width * height * 3  # 3 bytes for rgb24
-    frame_count = 10
-    start_frame = 2
+    frame_count = 15
+    start_frame = 3
 
     out = (
         ffmpeg.input(
@@ -700,6 +700,7 @@ def test_pipe():
             framerate=10,
         )
         .trim(start_frame=start_frame)
+        .setpts('N-{}+1'.format(start_frame))
         .output('pipe:1', format='rawvideo')
     )
 
@@ -716,9 +717,9 @@ def test_pipe():
         '-i',
         'pipe:0',
         '-filter_complex',
-        '[0]trim=start_frame=2[s0]',
+        '[0]trim=start_frame={}[s0];[s0]setpts=N-{}+1[s1]'.format(start_frame, start_frame),
         '-map',
-        '[s0]',
+        '[s1]',
         '-f',
         'rawvideo',
         'pipe:1',
@@ -739,9 +740,25 @@ def test_pipe():
     p.stdin.close()
 
     out_data = p.stdout.read()
+
     assert len(out_data) == frame_size * (frame_count - start_frame)
     assert out_data == in_data[start_frame * frame_size :]
 
+    return
+
+    if start_frame > 0 and len(out_data) < len (in_data):
+        # ffmpeg 4.x removes trimmed frames
+        assert len(out_data) == frame_size * (frame_count - start_frame)
+        assert out_data == in_data[start_frame * frame_size :]
+    else:
+        # ffmpeg 5.x duplicates trimmed frames
+        assert len(out_data) == len(in_data)
+
+        for fn in range(0, start_frame):
+            assert out_data[fn*frame_size] == in_data[start_frame * frame_size]
+
+        assert out_data[start_frame * frame_size :] == in_data[start_frame * frame_size :]
+
 
 def test__probe():
     data = ffmpeg.probe(TEST_INPUT_FILE1)