Fix transform sync in RigidBody*D::_body_state_changed

[mihe]

Fixes #84919.

This replaces the force_update_transform that was added as part of #84799 with the more crude solution of comparing the before-and-after of the global transform to determine whether we need to push any new updates to the physics server or not, thereby skirting around the problem of the global_invalid flag mentioned in #84856.

Note that this is not strictly necessary in the 3D bodies, as force_update_transform works just fine there, but I figured it's better to keep things consistent.

[akien-mga]

Thanks!