fix(desktop-gpui): keep the last frame of imported videos

[Theophilus2003]

Summary

Every video imported through the GPUI app comes out one frame short. Follow-up to #2575, where I found this while benchmarking imports.

Cause

H264Output::drain_packets in apps/desktop-gpui/src/import.rs writes the encoder's packets with duration = 0. libx264 (and VideoToolbox) don't fill it in. The mp4 muxer takes every other sample's duration from the next DTS, so only the final sample ends up with duration 0. The track and edit list therefore end one frame early:

• The bundled test-with-audio.mp4 fixture (25 frames, 1.00 s) imports as a file reporting 0.96 s. The mov demuxer flags its last frame as discard, so decoding it yields 24 frames.
• A 60 s 1080p60 import reports 59.983 s and a 30 s 4K30 import 29.967 s. Both lose one frame interval.

Fix

The encoder output stores one frame interval in its time base (frame_duration_ticks). Any packet the encoder leaves without a duration gets that interval before rescale_ts, so it's rescaled with the timestamps. Packets that already carry a duration are left alone.

Tests

• a_completed_conversion_keeps_every_source_frame_and_the_audio now also decodes the output: it must decode to all 25 frames and report a 1.00 s duration.
• Without the fix that test fails (left: 24, right: 25). With it, all import:: tests pass.
• rustfmt clean.

Confidence Score: 5/5

This PR appears safe to merge; no actionable issues were found.

Summary

This PR fills missing H.264 packet durations so imported videos keep their final frame.

• Imported videos keep their final frame.

_{Reviews (1) · Last reviewed commit: "fix(desktop-gpui): keep the last frame o..." · Reviewed by Greptile}

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[Theophilus2003]

Thanks for merging, @richiemcilroy

If it’s useful, #2586 is ready for review too: it makes the recording picker thumbnails up to 4.3x faster on Windows/Linux, with byte-identical output.