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