Fix center of mass calculations with welding.

Welding components uses precomputed weights for to determine center of mass for the actor. This can cause problems when users want to specify custom weights for certain components.

1. Add 2 actors to a level.
2. Attach the actors.
3. Set the parent actor to simulate physics.
4. Simulate the level, and observe the center of mass indicator (green sphere).
5. Change the mass of either mass by a significant amount.
6. Simulate the level, and observe the center of mass indicator.

Expected: Center of mass would shift closer to the center of the heavier actor.

Actual: Center of mass remains unaffected.