lavfi/WIP: port IVTC filters from vapoursynth.

TODO
 - split filters in 2 commits
 - fieldmatch: test properly
 - fieldmatch: make sure modes are working as expected
 - fieldmatch: flushing
 - decimate: fix crashes
 - decimate: document options
 - decimate: flushing
 - decimate: check memleaks
 - write documentation
 - version bumps
 - Changelog