lavc/elbg: rename ff_ symbols to avpriv_, so they can be used in shared libs

In particular, allows the use of elbg in other FFmpeg libraries, required
by the pending elbg filter.
lavfi: add elbg filter
lavd/xv: fix memory leak

Results of XvQueryAdaptors have to be freed with XvFreeAdaptorInfo.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
doc: remove soc.txt

The file was written in 2006, it was never updated nor really used as
reference for SOC-like programs.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
avformat/mmst: propagate error code from avio_put_str16le()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>