The first of these binds inputs of complex filtergraphs to demuxer
streams (with a misleading comment claiming it *creates* complex
filtergraphs).
The second ensures that all filtergraph outputs are connected to an
encoder.
Merge them into a single function, which simplifies the ffmpeg_filter
API, is shorter, and will also be useful in following commits.
Also, rename misleadingly-named init_input_filter() to
fg_complex_bind_input().
7b51523f12