This patch is analogous to 20f9727018:
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>
It is useful for testing/debugging and will also be used as the default
filter in the following commit adding pre-decode filtering to avoid
having a separate non-filtered codepath.