feat(FFmpeg): add V4L2 zero-copy support

[GenshinImpactStarts]

Description

This PR adds two FFmpeg patches required for Sunshine's upcoming V4L2 M2M hardware encoder support (H.264/HEVC/AV1) on Linux.

To achieve true zero-copy, Sunshine needs to import DMA-BUFs directly into FFmpeg's V4L2 output buffers. FFmpeg's current v4l2_m2m implementation uses a memcpy path, so two patches are needed to expose the required private headers/symbols to downstream consumers and adapt the buffer handling for Sunshine.

Patch details

• 01-expose-function.patch: Exposes static/private functions so consumers can explicitly stop the encoder, reclaim output buffers, and set V4L2 extended controls.
• 02-specify-buffer.patch: Adds additional fields and conditional logic to work with buffers that Sunshine has already written.

Scope and behavior

• Applies by default only on Linux and FreeBSD amd64.
• Explicitly disabled on Windows, macOS, and FreeBSD aarch64.
• Zero-copy is an opt-in per-context switch, so existing FFmpeg consumers remain unaffected.

Screenshot

Issues Fixed or Closed

Roadmap Issues

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semicolons, etc.)
• refactor: Code change that neither fixes a bug nor adds a feature
• perf: Code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
• Code has been self-reviewed
• Code has been commented, particularly in hard-to-understand areas
• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

AI Usage

See our AI usage policy.

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud