avfiltergraph: try to reduce format conversions in filters.

Current code, with a filterchain such as
(input - yuv411) -> (scale - any) -> (sink - any)
will result in yuv420 being chosen for the second link, which is clearly
not right.

This commit attempts to improve in the following way:
repeat until convergence:
  loop over all filters
    find input link with exactly one format
    force this format on all output links of the same type (if possible)
