mandelbrot: add outer coloring method showing bailouted z

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vsrc_mandelbrot: Mandel morphing support

Example: ffplay -f lavfi "mandelbrot=outer=outz:morphamp=1:end_scale=2:maxiter=100"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffplay: port avfilter buffersrc fps code from ffmpeg

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
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
 - fieldmatch: test properly
 - fieldmatch: make sure modes are working as expected
 - decimate: document AVOptions
 - decimate: flushing
 - decimate: setfps
 - check memleaks
 - write documentation
 - version bumps
 - Changelog