🍒 h264vaapi: Use the proper getter for getting the hardware context
The hw_frames_context was moved out of AVFilterLink for FFmpeg 7.1. Apparently there is actually a proper getter for getting the hw_frames_ctx of a filter context, so use that instead so things keep compiling with 7.1.
While at it, add a comment explaining why we need this in the first place.
(cherry picked from commit 5ff964b6)
Co-authored-by: Arjen Hiemstra ahiemstra@heimr.nl