Blender Git Loki

Motion paths: Use minimal possible dependency graph

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