CollisionGroup::Noncollidable fix

[eundersander]

Motivation and Context

Fix to ensure CollisionGroup::Noncollidable has a mask of 0 and thus collides with nothing.

The previous code used a mask of -1, with the result that a Noncollidable would collide with another Noncollidable.

How Has This Been Tested

Ad hoc local testing

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[aclegg3]

Good catch!
We should add a test, then ship it.
Adding the following at the end of test_collision_groups() should do.

# test Noncollidable
cube_obj1.override_collision_group(cg.Noncollidable)
assert not cube_obj1.contact_test()
assert not cube_obj2.contact_test()