This commit brings back motion path calculations working on same level
as 2.7x. Caveats:
* You cannot actually see anything, as all the drawing code for motion
paths was removed and not reimplmented in 2.8 draw engines yet
* This doesn't actually use a separate depsgraph instance for fast
background calculations yet. That's a separate topic, and will require
further investigation (probably after code quest, according to original
planning).
* The work here *does* however pave the path forwards towards fully separate
COW evaluation though. We only need to stub in a different depsgraph instance
here to get things working.
e7eee7764a