recorder: pad odd source dimensions before libx264 encode

[Sayan-]

Summary

libx264 with yuv420p requires both width and height to be divisible by 2 (4:2:0 chroma subsampling). When the captured display has an odd vertical (or horizontal) pixel count, ffmpeg fails to open the encoder with width/height not divisible by 2 and exits almost immediately — but Start() only waits 250ms for a startup error, so the API still returns 200 and the caller is left with a zero-byte mp4.

This change adds a -vf pad=ceil(iw/2)*2:ceil(ih/2)*2 filter so odd source dimensions are padded by a single pixel of black before encode. Even dimensions are unchanged.

Repro

$ ffmpeg -f lavfi -i color=red:size=641x481:rate=5 -t 1 -c:v libx264 -pix_fmt yuv420p -y out.mp4
[libx264] width not divisible by 2 (641x481)
[enc:libx264] Could not open encoder before EOF
$ ls -la out.mp4
-rw-r--r-- 1 user user 0 ... out.mp4

With the filter:

$ ffmpeg -f lavfi -i color=red:size=641x481:rate=5 -t 1 -vf "pad=ceil(iw/2)*2:ceil(ih/2)*2" -c:v libx264 -pix_fmt yuv420p -y out.mp4
$ ffprobe out.mp4
width=642 height=482

Test plan

• go build ./...
• go test ./lib/recorder/... -v (existing tests pass, new TestFFmpegArgs_PadsOddDimensions covers the args)

---

Note

Medium Risk
Touches ffmpeg encoding arguments for all recordings, which could subtly affect output dimensions/compatibility if the filter order or padding behavior differs across inputs/platforms.

Overview
Prevents recordings from failing on odd-sized displays by adding -vf pad=ceil(iw/2)*2:ceil(ih/2)*2 to the ffmpeg output args before libx264/yuv420p, ensuring width/height are even.

Adds TestFFmpegArgs_PadsOddDimensions to assert the new -vf filter is present in the generated argument list.

^{Reviewed by Cursor Bugbot for commit 72b0e42. Bugbot is set up for automated code reviews on this repo. Configure here.}

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