Correctly set mass for a rigid body with custom inertia and center of mass

[Crimzoth]

See #78750 for issue description.

Removed erroneous check, which caused _inv_mass not to be calculated when RigidBody2D or RigidBody3D used both custom center of mass and custom inertia.

The only purpose of the check for (calculate_inertia || calculate_center_of_mass) appears to be to prevent adding bodies to the godot_space's mass_properties_update_list . On elements of this list GodotBody3D::update_mass_properties() is then executed.
At the first glance it appears that if calculate_inertia == false and calculate_center_of_mass == false function GodotBody3D::update_mass_properties() would do nothing and this check in GodotBody3D::_mass_properties_changed() is valid.
This is not entirely true. GodotBody3D::update_mass_properties() is also used to calculate _inv_mass, which would be left at default value if the body was omitted from mass property update and cause incorrect physics behavior as described in #78750.

By removing the check for (calculate_inertia || calculate_center_of_mass) it can be ensured that _inv_mass is always calculated. At the same time GodotBody3D::update_mass_properties() already contains checks for the above parameters, meaning that if calculate_inertia == false or calculate_center_of_mass == false no unnecessary calculations will be executed. Checking for these parameters twice is not needed.

• Bugsquad edit, fixes: Mass not set properly for RigidBody3D with custom inertia and center of mass #78750

[AThousandShips]

Please update the title of the PR and the message of the commit to be more descriptive of what is actually being done

[AThousandShips]

If this is the correct solution it should probably be done for 2D as well for completeness

[Crimzoth]

Yes, I've just checked and it affects 2D as well. I've added another commit for RigidBody2D.

[YuriSizov]

Please note that PRs should only contain one commit as a rule, so your commits need to be squashed (with some common message for both changes).

[Crimzoth]

Please note that PRs should only contain one commit as a rule, so your commits need to be squashed (with some common message for both changes).

Squashed into single commit

[fire]

@aaronfranke You might be interested in center of mass prs.

[wmigor]

I am making an airplane simulator. I am waiting for this fix

[aaronfranke]

Not 100% my area of expertise but this looks good to me. It makes sense that this code should always run even when the user sets their own values. It also makes sense why this bug wasn't discovered earlier, most people do not set custom inertia so it's unlikely that this bug would be encountered often.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉