Blender Git Loki

Fluid Particles: fix threading crash with viscoelastic springs.

As correctly pointed out by a comment in the code, adding
new springs wasn't thread safe, and caused crashes.

Fix by buffering new springs in intermediate thread-local
arrays, which are flushed on the main thread. This is valid
because the new springs are not used until the next sim step.

Differential Revision: https://developer.blender.org/D6133