fftools: Switch to const AVCodec * where possible

fftools: Switch to const AVCodec * where possible

The obstacle to do so was in filter_codec_opts: It uses searches
the AVCodec for options via the AV_OPT_SEARCH_FAKE_OBJ method, which
requires using a void * that points to a pointer to a const AVClass.
When using const AVCodec *, one can not simply use a pointer that points
to the AVCodec's pointer to its AVClass, as said pointer is const, too.
This is fixed by using a temporary pointer to the AVClass.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>