Guess the duration before converting video and write guessed duration
	into flv header.
	Patch by Justin Johnson, justin D johnson3 A gmail

The change to ffmpeg.c is wrong as input and output files arent connected 1:1
(i seem to have missed that)
Also it can segfault see ([FFmpeg-devel] rtp streaming x264+audio issues (and some ideas to fix them))

Originally committed as revision 21652 to svn://svn.ffmpeg.org/ffmpeg/trunk