ffoutput will overflow memory when used for large videos

[asquared]

The "ffoutput" program will constantly read data from ffmpeg even before it is needed. This can result in large memory buffers being allocated. Potential fixes include using separate ffmpeg processes for video and audio decoding, or blocking the ffmpeg process when buffers exceed a preset size. Separate processes are likely to work best, in light of the fact that ffmpeg audio decoding can be blocked if nothing reads from the video pipe, and vice versa. (Thanks go to lpelay for initially reporting this.)