Self collision does not work properly for tree structure

[jslee02]

For example, if we test self collision with hand like structure, the contact handling does not work properly when two fingers are colliding.

[karenliu]

The bug was in UpdateNBMatrices(). When there is a self collision, the two colliding bodies fill in the same block of matrix N and matrix B. In the original code, body 2 will override body 1's value. To fix the bud, we only need to modify "=" to "+=" when updating body 2's block.

[jslee02]

This bug is fixed at master branch (a696486) and release-3.0 branch (3de01ab)