Refactor: Convert RGBA to RGB with bounds checking in MP4 export

[seer-by-sentry]

Fixes CAP-DESKTOP-J2. The issue was that: Screenshot generation incorrectly slices padded GPU texture data, causing memory corruption detected by macOS VFX framework during Metal texture processing.

• Converts RGBA frame data to RGB during MP4 export.
• Implements bounds checking to prevent memory access violations when processing frame data, ensuring robustness.
• Uses min to safely handle cases where row length is shorter than expected.
• Ensures chunk has at least 3 bytes (RGB) before accessing.

This fix was generated by Seer in Sentry, triggered by Richie. 👁️ Run ID: 2575842

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