fix(linux/vulkan): preserve host aspect ratio in encoder output

[hebo6]

Description

Fixes a behavior inconsistency between the VAAPI and Vulkan encoders on Linux: when the encoder target resolution differs in aspect from the captured source, the VAAPI path letterboxes the source into the encoded frame while the Vulkan path stretches it to fill.

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

• 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

[neatnoise]

As the original author of the Vulkan encoder path — this looks correct and clean. Single dispatch, no extra passes or sync, negligible overhead.

A few notes:

1. rgb2yuv.comp L43-47: The black value computation assumes range_y/range_uv are (1,0). Worth a comment noting this assumption so it doesn't break silently if those values change in the future.

2. vulkan_encode.cpp L115-116: Push constant struct layout changed (fields inserted in the middle) — fine since both C++ and GLSL are updated together, but worth being aware of for any future out-of-tree shader variants.

3. vulkan_encode.cpp L290-293: The & ~1 rounding on eff_w/eff_h and offsets could theoretically leave a 1-pixel gap if frame dimensions are odd. Might want to assert or clamp that dst_offset + eff_size <= frame_size.