This patch is analogous to 20f97270:
It hides the internal part of AVBitStreamFilter by adding a new
internal structure FFBitStreamFilter (declared in bsf_internal.h)
that has an AVBitStreamFilter as its first member; the internal
part of AVBitStreamFilter is moved to this new structure.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>