lavfi: drop now unused shorthand field from AVFilter.
lavfi/testsrc: make use of AV_OPT_TYPE_DURATION

Simplify.
vf_mp: mp buffers are not compatible with the reference count system

We thus must copy each frame on the vf_mp output.

This fixes artifacts with "ffplay -threads 1 dnxhdconv.mov -vf mp=eq2=1:1"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>