ffmpeg: fix MFX session leak in QSV

[cgutman]

Description

This fixes a bug in the QSV hwdevice context that causes MFX sessions (and thus the ID3D11Device and associated resources) to be leaked every time we open an encoder session.

This can't go upstream as-is due to conflicting changes in FFmpeg master, but I plan to submit a patch upstream too.

Screenshot

Issues Fixed or Closed

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

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must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated