[p5.strands] Auto-return input objects if omitted

[davepagurek]

Right now, many (but not all!) shader hooks involve taking in a struct, modifying it, then returning it, e.g.:

  getObjectInputs((inputs) => {
    inputs.position.x += 10;
    return inputs;
  })

We could make it so that if the input and return types are the same, if your hook callback doesn't already return something, we automatically return the input object, making this ok:

  getObjectInputs((inputs) => {
    inputs.position.x += 10;
  })