DVDOverlayCodecFFmpeg: Fix for distorted subtitles in 3D modes

[TheMontezuma]

See: https://github.com/xbmc/xbmc/issues/16362

Description

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of change

• [X] 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 change)

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

[TheMontezuma]

I hope that this is the appropriate KODI branch for future LibreELEC releases.