Blender Git Loki

Optimization: Use inertial solve for pre-collision

Instead of doing a full implicit pre-collision solve, do a simple
inertial solve (avance using velocities from previous step), this saves
time by doing one less solve per step, and also improves resting
stability by eliminating the influence of external forces disturbing the
mesh in contacting areas.

This improves general cloth performance by about 30-40% in relation to
the new cloth code, and improves performance by about 3 times in
relation to the original cloth code. This also makes collisions more
reliable by removing the need for positional interpolation, and thus
decreases the number of subframes required for a successful solve.