ext/ffmpeg/gstffmpegenc.*: Instead of allocating 512KB buffers and then setting BUFFER_SIZE to what ffmpeg produced, ...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegenc.c: (ffmpegenc_setup_working_buf),
(gst_ffmpegenc_chain_video), (gst_ffmpegenc_flush_buffers),
(gst_ffmpegenc_change_state):
* ext/ffmpeg/gstffmpegenc.h:
Instead of allocating 512KB buffers and then setting BUFFER_SIZE
to what ffmpeg produced, we're better off by far to use a single
working buffer and copy data out into neat little buffers.
This prevents exhorbitant virtual memory wastage in the form
of allocated but untouched buffers.