Add YADIF (2x) deinterlacing

The Channels app has been developing a version of YADIF2x which is optimized for Apple GPUs, using Metal.framework.

We decided to open-source this work so everyone can benefit. The Metal-optimized deinterlacer is now included as part of FFmpeg, and can be used by apps like Infuse once they upgrade to the upcoming FFmpeg 5.0 release.