Blender Git Loki

Resolve the opposite vector ambiguity in Damped Track constraint.

Damped Track by specification attempts to arrive at the desired
direction via the shortest rotation. However with opposite vectors
there are infinitely many valid 180 degree rotations. Currently
it gives up and does nothing.

I think that it would be more reasonable to resolve the ambiguity
arbitrarily, so that Damped Track won't have a weird dead zone.
To make it more predictable I use a local axis.

In addition, the singularity area vicinity has some floating
point precision problems that result in significant jitter.
This applies workarounds for two causes of instability.

Differential Revision: https://developer.blender.org/D3530