Samples: audio timeline gaps and overlaps reach the encoder in its frames

The gap and overlap fixtures cut timestamps with aselect and asetpts.
ffmpeg 8 regroups the filter's audio into encoder frames and gives each
the timestamp of its first sample, so on ubuntu-latest the gap fixture
came out with no gap and the timeline test found the sine where the
seek expects silence. Those fixtures' source frames are now exactly
the encoder's (960 samples for Opus, 64 for Vorbis), which ffmpeg
hands on as they are, timestamps included.

samples/cores/webm_player/audio_timeline_fixtures.py

@@ -12,10 +12,17 @@ from audio_movie_reference import aac_matroska_pcm
 out = Path(sys.argv[1] if len(sys.argv) > 1 else "fixtures")
 out.mkdir(parents=True, exist_ok=True)
 
-def make(name, *, delay=0, duration=4, audio_duration=2, codec="libopus", filter=None, mp4=False, trim=0):
+# An encoder's frame length in samples. A source whose frames are this
+# long reaches the encoder as they leave the filter, timestamps included:
+# ffmpeg 8 otherwise regroups them into encoder frames and
+# keeps only the first one's timestamp, closing a gap or an overlap.
+FRAME = {"libopus": 960, "libvorbis": 64}
+
+def make(name, *, delay=0, duration=4, audio_duration=2, codec="libopus", filter=None, mp4=False, trim=0, framed=False):
+    frames = f":samples_per_frame={FRAME[codec]}" if framed else ""
     args = ["ffmpeg", "-v", "error", "-y", "-f", "lavfi", "-i",
             f"testsrc2=size=64x48:rate=12:duration={duration}", "-itsoffset", str(delay),
-            "-f", "lavfi", "-i", f"sine=frequency=440:sample_rate=48000:duration={audio_duration}"]
+            "-f", "lavfi", "-i", f"sine=frequency=440:sample_rate=48000:duration={audio_duration}{frames}"]
     if filter:
         args += ["-af", filter]
     if mp4:
@@ -108,9 +115,9 @@ for delay in (0, 1, 2):
     make(f"timeline-{delay}.webm", delay=delay)
 for codec in ("libopus", "libvorbis"):
     make(f"timeline-gap-{codec}.webm", duration=5, audio_duration=4,
-         codec=codec, filter=r"aselect=not(between(t\,1\,2))")
+         codec=codec, filter=r"aselect=not(between(t\,1\,2))", framed=True)
 make("timeline-overlap.webm", duration=5, audio_duration=4,
-     filter=r"asetpts=PTS-0.5/TB*gte(T\,2)")
+     filter=r"asetpts=PTS-0.5/TB*gte(T\,2)", framed=True)
 for delay in (0, 1):
     make(f"timeline-{delay}.mp4", delay=delay, codec="aac", mp4=True)