This avoid a crash with in avfilter_merge_formats() in case one of the
filter formats list has multiple time the same entry.
Thanks to Mina Nagy Zaki for helping figuring out the issue.
Make the mp wrapper accept the syntax mp=filter=params as alternative
to mp=filter:params. The alternative syntax is sligthly more readable
and should simplify copy&paste of MPlayer filter strings to the mp
filter.