Add MassPropertyPlugin

[Jondolf]

Objective

Logic related to mass properties is currently a bit scattered around (PreparePlugin, ColliderBackendPlugin...), and there is also no way to opt out of automatic updating of mass properties. It would be nice to encapsulate this logic inside a single plugin, especially as we rework mass properties (#499) and add more APIs and functionality for working with mass.

This also contributes to the goal of removing PreparePlugin, which is currently full of various miscallaneous and unrelated logic.

Solution

Add a MassPropertyPlugin. It updates mass properties of rigid bodies when colliders are added or removed, or when their ColliderMassProperties are updated. It also logs warnings for invalid mass properties, and updates GlobalAngularInertia.

Previously, the logic for updating mass properties based on ColliderMassProperties changes was heavily tied to the ColliderMassProperties update logic in ColliderBackendPlugin, because it required knowing both the previous and new mass properties of the collider. Same for mass property updates caused by adding or removing colliders.

Now, it doesn't directly change the body's mass properties, but instead triggers an OnColliderMassPropertiesChanged event that the MassPropertyPlugin (and users) can react to. This makes the collider logic only responsible for managing ColliderMassProperties, and the logic for rigid bodies is self-contained.

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Migration Guide

The MassPropertyPlugin is now needed to update mass properties automatically based on attached colliders. Most apps won't need to add it manually, as it is included in the PhysicsPlugins plugin group by default.