lavfi: rename decimate to mpdecimate.

The next commit will introduce a proper decimation filter to be used
along with the field matching filter. To avoid confusion with this
filter which has currently a very limited usage (and will not work
properly with the fieldmatching filter), the new decimation filter will
take the decimate name, and this filter is renamed to mpdecimate.

TODO: bump minor
lavfi/WIP: port IVTC filters from vapoursynth.

TODO
 - complete documentation
 - version bumps
 - Changelog