Stop using deprecated FFMPEG calls in the code.

This allows the port to be built with the upcoming FFMPEG 3.x series while
still maintaining compatibility with FFMPEG 2.x that is currently in the ports
tree.

I don't think PORTREVISION needs to be bumped here, as both the old and new
calls exist in FFMPEG 2.x, and when FFMPEG 3 lands there will be a PORTREVISION
bump anyway.

PR: 214184
Submitted by: Matthey Rezny <matthew@reztek.cz>