[ffmpeg] Fix stereo upmixing

[sundermann]

Description

AVChannel was compared with channel masks instead of the AVChannel enum. This resulted in only the center and front right speaker being remapped in the matrix.

Motivation and context

Fixes #25028

How has this been tested?

MacOS

What is the effect on users?

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)
• 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:

• 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

[fritsch]

Very good catch. I fear we have same issue somewhere else.

[sundermann]

Only caught it by accidence because it was next to the code that broke channel remapping in ffmpeg 7.0. I did a quick grep through the code and the other direct uses of the AV_CH_* macros seem fine.

[sundermann]

Jenkins build this please