[Bullet] Bullet physics adds the default damping values instead of setting them

[e344fde6bf]

Godot version:
3.2.3.stable

OS/device including version:
N/A

Issue description:

Bullet adds the default linear/angular damping values instead of setting them. This issue doesn't occur when using Godot physics backend.

Bug is probably from here:

godot/modules/bullet/rigid_body_bullet.cpp

Lines 974 to 979 in dd1881a

	// Add default gravity and damping from space.
	if (!stopped) {
	newGravity += space->get_gravity_direction() * space->get_gravity_magnitude();
	newLinearDamp += space->get_linear_damp();
	newAngularDamp += space->get_angular_damp();
	}

Steps to reproduce:

• Create a rigid body and set its linear damping value to 0
• Inside physics process PhysicsServer.body_get_direct_state(get_rid()).total_linear_damp is now set to 0.1 (this is the default linear damping set inside project settings)

Minimal reproduction project:
bullet-damping-bug.zip

[Calinou]

Related to #42842.

[JestemStefan]

I encounter this today. I'm not sure if it's the bug, but for sure it's inconsistent with what is written in docs

According to it any value different then -1 should override default damp setting, but in reality default damp value is added to local damp value (if local damp is bigger then 0).

The behaviour mentioned in the docs makes more sense

[Seubmarine]

We should change the doc or create a pull request were we change that