perf(windows): skip the webcam frame copy when its sequence has not changed

[EtienneLescot]

The video writer no longer copies the webcam frame when it already holds it. With a 30 fps camera and a 60 fps screen, that removes about 30 full-frame copies a second from the thread that paces the screen.

Fixes #934
Part of #920

Change

• copyLatestFrame takes the caller's last seen sequence and returns false without copying when it has not changed.
• Both backends (Media Foundation and DirectShow) route through one inline rule, snapshotWebcamFrame, next to WebcamFrameSnapshot.
• main.cpp: only the two copyLatestFrame calls in the webcam blocks change. The sequence comparison moved into the call.

Measured

• New webcam_snapshot_test, wired into CMake and build-windows-wgc-helper.mjs: an unchanged sequence returns false and copies nothing.
• It fails on the old rule: with the sequence check removed, 2 assertions fail (unchanged sequence reports nothing new, unchanged sequence copies nothing).
• node scripts/build-windows-wgc-helper.mjs: every target builds, all four native test executables pass.

Pending (maintainer, real take)

• storeMs / readSampleMs in INFO: Webcam capture loop ended, and the helper's CPU, before and after.
• Not changed: a frame rejected as not yet visible (camera warm-up) is still copied on each tick until a visible one arrives, as before.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 345e225 and 975fb58.

📒 Files selected for processing (8)

• electron/native/wgc-capture/CMakeLists.txt
• electron/native/wgc-capture/src/dshow_webcam_capture.cpp
• electron/native/wgc-capture/src/dshow_webcam_capture.h
• electron/native/wgc-capture/src/main.cpp
• electron/native/wgc-capture/src/webcam_capture.cpp
• electron/native/wgc-capture/src/webcam_capture.h
• electron/native/wgc-capture/src/webcam_snapshot_test.cpp
• scripts/build-windows-wgc-helper.mjs

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