ffmpeg: store value of -filter and -filter_script per-stream option in OutputStream struct

Avoid the need for multiple potentially inconsistent access operations,
slightly factorize code.
ffmpeg: generate more specific error message in case of inconsistent filtering options

In particular, distinguish if the option was -filter or -filter_script.

Suggested-by: Nicolas George
libavf/avienc: store language metadata for XSUB streams

Formatted in such a way that DivX certified players can decode it.
Verified on Sony Playstation 3 and Philips DVP3380.

Fixes ticket 2385

Signed-off-by: Erik Olofsson <eaj.olofsson@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>