This commit includes several performance, stability, and reliability
improvements to cloth collisions.
Most notably:
* The implementation of a new self-collisions system.
* Multithreading of collision detection.
* Implementation of single sided collisions and normal overrides.
* Replacement of the `plNearestPoints` function from Bullet with a
dedicated solution.
Further, this also includes several bug fixes, and algorithmic
improvements.
Reviewed By: brecht
Differential Revision: http://developer.blender.org/D3712
0666ece2e2