Fix vector reflect() mutating surface normal arg (#7088)

[nbogie]

Resolves #7088

Changes:

Added unit tests to check the reflect functions (static and instance) don't alter the given surfaceNormal vector.

Changed the common p5.Vector reflect function so that the given surfaceNormal vector is not mutated unexpectedly.

Did so by making a local copy of the input vector:

reflect(surfaceNormal) {
    const surfaceNormalCopy = p5.Vector.normalize(surfaceNormal);
    return this.sub(surfaceNormalCopy.mult(2 * this.dot(surfaceNormalCopy)));
}

Screenshots of the change: n/a

PR Checklist

• [x] npm run lint passes
• [ ] [Inline documentation] is included / updated - n/a (bugfix)
• [x] [Unit tests] are included / updated

Note: eslint passes but emits one warning.

p5.js/src/core/reference.js:0:0: File ignored because of a matching ignore pattern. Use "--no-ignore" to override. [Warning]

misc implementation notes

Also considered this alternative implementation using the static methods to avoid a local copy of the vector, but I think it turns out harder to read, and requires the normalisation calculation happen twice.

reflect(surfaceNormal) {
    return this.sub(
      p5.Vector.mult(
        p5.Vector.normalize(surfaceNormal),
        2 * this.dot(p5.Vector.normalize(surfaceNormal))
      )
    );
  }

[limzykenneth]

Thanks @nbogie. I think having a local copy of the vector is the better approach as it is also more memory efficient.