Remove the inconsistent, duplicated, and insufficient scale filter
insertion code, and do it in one place instead. This also compensates
for the earlier removal of vf_match_csp() (which was in fact duplicated
code).
The algorithm to determine where to insert a filter etc. is probably the
same, though it also comes with some changes that should make debugging
easier when trying to figure out why a chain is failing to configure.
Add an "in" pseudo filter, which makes insertion of conversion filters
easier. Also change the vf->reconfig signature. At a later point, I'll
probably change format negotiation such that the generic filter code
will choose the output format, so having separate in and out params will
be useful.