PSA: i420_buffer.h moving to a new target

[Patrik Höglund]

Hi!

In two weeks (Nov 27), I am going to land https://webrtc-review.googlesource.com/c/src/+/22640, which moves api/i420_buffer.h out of the build target api:video_frame_api and into a new target.

After the change, there will be

• api:video_frame_api
  
• api:video_frame_api_i420
  

Depend on the latter if you include i420_buffer.h. api:video_frame_api_i420 is already available but currently only contains the header, so you can start depending on it now. The .cc will move into the i420 target on Nov 27.

See bug 7504 for context.

BR,

/ P