Damping values are now non-negative.
Add new properties linear_damp_mode and angular_damp_mode to set the way
RigidDynamicBody and PhysicalBone (2D & 3D) use damping values.
It can now be Combine (default) to add to the default/areas, or Replace
to override the value completely (current behavior).
Same logic as what was done in 3D, applied to 2D center of mass.
Also did some minor cleanup in 3D and fixed center of mass transform
during the first frame after teleporting a dynamic body.