This change makes it so motion paths are using minimal possible
dependency graph which is sufficient to evaluate required motion
path targets.
Disclaimer: granularity is done on ID level, but it is possible
to go more granular if really needed.
Brings time down to 0.5 sec when updating motion path for the
Rain animation file used for benchmarks in the previous commits.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D5874
The idea is to only update parts of motion paths which lies between
keyframes around changed frame.
The changed frames are considered to be from second previous to
second next one to work correctly with bezier handles interpolation.
Brings updates in the Rain test animation file from around 11 sec
down to 1 sec.
Having this in blenkernel caused bad level calls to bf_editors_anim,
causing tests that use 'blenkernel' to require almost all libraries.
(complicating gtest linking & causing large binaries).
