`preset` object of particles will be changed in test case

[MasatoMakino]

Expected behavior

preset object will not be changed in the test.

Actual behavior

Vector3D objects in preset object will be changed in the test.

How to reproduce

Output preset before and after particle.reset().

https://github.com/creativelifeform/three-nebula/blob/7e0b2dc9afefc4838a22475d6cca91e513ad9812/test/core/Particle.spec.js#L112-L115

  it("should reset the particle's clearable properties and return the particle", done => {
    const particle = new Particle(preset);
    console.log(preset); // <- velocity: Vector3D { x: 1, y: 2, z: 2 } 
    const reset = particle.reset();
    console.log(preset); // <- velocity: Vector3D { x: 0, y: 0, z: 0 }

Impact

Currently this issue does not affect us, but it may damage our tests in the future.

Cause

https://github.com/creativelifeform/three-nebula/blob/7e0b2dc9afefc4838a22475d6cca91e513ad9812/src/core/Particle.js#L232-L237

Particle instances and preset share the same Vector3D object. Vector3D.set() impacts preset.

Suggestion

https://github.com/creativelifeform/three-nebula/blob/7e0b2dc9afefc4838a22475d6cca91e513ad9812/test/core/fixtures/particle.js#L4

Change preset object to getPreset() function. If the preset object is instantiated for each test cases, the impact will not go over the test case.