lavfi/WIP: port IVTC filters from vapoursynth.

TODO
 - fieldmatch: test properly
 - fieldmatch: make sure modes are working as expected
 - decimate: document AVOptions
 - decimate: flushing
 - decimate: setfps
 - check matching input sources parameters
 - check memleaks
 - write documentation
 - version bumps
 - Changelog