vp9: use superframe split BSF

webm usually has invisible superframes merged with normal frames.
(vpxenc muxes them in this form, which is evidence enough that this is
the standard webm packet format. It's rather unclear whether ffmpeg is
even allowed to remux them with split packets.)

The vp9 decoder needs them to be in separate packets for multithreading
to work. Add the BSF to the decoder, so the conversion happens
automatically.

This contains the important part of fa1749dd34c55fb9, which
was apparently skipped in commit d417e95af76. This restores Libav API
compatibility.

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