fix(video): remove explicit frame flushing to avoid segfault due to racecondition

[Kishi85]

Description

This PR aims to fix #4943 by removing the frame flushing on ~avcodec_encode_session_t() which can result in a segfault due to FFmpeg hw_base_encode expecting ctx->pic_end to never be null but Sunshine seems to break that assumption in this case sometimes (likely due to a racecondition somewhere in FFmpeg as it segfaults on the hw level).

From a logical standpoint it's not strictly necessary to explictly flush the frames here as the avcodec_ctx used to flush the frames is by itself reset and destroyed in the next statement (therefore FFmpeg cleaning up here by itself?). This restores the behaviour back to what was in use for FFmpeg versions before 8.0.

I've tested this for a few days and cannot see any adverse effects from removing this section but as this change is a global one it should be thoroughly tested before merging.

Screenshot

Issues Fixed or Closed

• Fixes Coredump with SIGSEGV when switching displays #4943

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

[codecov]

Bundle Report

Bundle size has no change ✅