avconv: decouple configuring filtergraphs and setting input parameters

Currently, calling configure_filtergraph() will pull in the input
parameters from the corresponding decoder context. This has the
following disadvantages:
- the decoded frame is a more proper source for this information
- a filter accessing decoder data breaks proper layering

Add functions for explicitly sending the input stream parameters to a
filtergraph input - currently from a frame and a decoder. The decoder
one will be dropped in future commits after some more restructuring.