Allow randomRamp particles to use different arrays

[gemedet]

This PR adds a boolean flag to randomRamp, allowing the randomization to be per particle.

[twostraws]

I think I understand what this trying to do, but the name preselect is really throwing me off. What alternatives are there?

[gemedet]

@twostraws I also considered the names fixed and shared, let me know if you have another idea.

Alternatively, we could just add a separate case, such as:

// Select one random color array for all particles
case randomRamp(_ colors: [[Color]])

// Select a random color array for each particle
case multiRamp(_ colors: [[Color]])