* Allows creating a GDExtension based 3D Physics Server (for Bullet, PhysX, etc. support)
* Some changes on native struct binding for PhysicsServer
This allows a 3D Physics server created entirely from GDExtension. Once it works, the idea is to port the 2D one to it.
Regression fix, gravity was accumulated between frames after some
changes around area gravity calculation.
Also got rid of unused member and method in soft body class.