avfilter/avfiltergraph: add logging for filter formats

avfilter/avfiltergraph: add logging for filter formats

There is no convenient way, from the command line, to figure out which
formats a filter actually supports. This commit changes that by adding a
log output, at debug level, to simply print the list of formats each filter
advertises on its links, before any negotiation.

Furthermore, we can use the exact same helper function to also print out the
corresponding filter links when there is an error during format negotiation.

We need to use AV_BRINT_SIZE_UNLIMITED because the default format list for
filters like vf_scale is about 1700 characters long, significantly larger than
the the 1 kB default buffer.