run-ffmpeg uses run-audio-buffer function instead of with-audio-buffer macro

function has better composibility for hot swapping with video buffer type
run-audio-buffer accepts ffmpeg-env as parameter

ffmpeg-env can contain information for starting both an audio and video buffer
packages ring-buffer exports with-ring-buffer
inherit video-params by default have buffer of size 120 frames with 5 periods
run-audio-buffer dynamically chosen from media-type member of ffmpeg-env