[FFmpeg-user] memory leak with vaapi decode since ffmpeg_vaapi: Convert to use hw_frames_ctx only

[Andy Furniss]

First reported on mpv github issues.

Since below ffmpeg -hwaccel vaapi -i sommething-h264 -f null -

will steadily eat memory.

commit 542a65d0b33abf81e5087ec9142bb11a54f23cde
Author: Mark Thompson <sw at jkqxz.net>
Date:   Sun Jul 31 22:51:45 2016 +0100

     ffmpeg_vaapi: Convert to use hw_frames_ctx only

     Most of the functionality here has moved into lavc.

     (cherry picked from commit 3e8651a7ccd8e89cc2f162cf614a3c9f7f4d9fcf)