VideoPlayer: add setting for using VAAPI AVC

[smp79]

Description

This adds "Use AVC VAAPI" setting. I always thought it was already there but apparently it wasn't.

Motivation and context

Makes it possible to disable VAAPI hw acceleration for AVC.

How has this been tested?

Build and runtime tested on Intel TGL + LibreELEC.

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• [ X] Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

Checklist:

• [X ] My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[howie-f]

runtime tested. works.
i can disable ff-h264-vaapi (HW) and switch to ff-h264 (SW)