avformat: only allow a single bitstream filter when muxing

avformat: only allow a single bitstream filter when muxing

Current muxers only use a single bitstream filter, so there is no need to
maintain code which operates on a list of bitstream filters. When multiple
bitstream filters are needed muxers can simply use a list bitstream filter.

If there is a use case in the future when different bitstream filters should be
added at subsequent packets then a new API possibly involving reconfiguring the
list bitstream filter can be added knowing the exact requirements.

Signed-off-by: Marton Balint <cus@passwd.hu>