avconv: decouple configuring filtergraphs and setting output parameters

Currently, a filtergraph will pull in the output constraints from its
corresponding decoder context, which breaks proper layering. Instead,
explicitly send the constaints on the output parameters to the
filtergraph.

This is similar to what is done for filtergraph inputs in
30ab4c51a180610d9f1720c75518d763515c0d9f