Function sortByVisibility

Sorts the provided indices by visibility score (descending order).

Visibility is computed as: linear_opacity * volume where:

• linear_opacity = sigmoid(opacity) = 1 / (1 + exp(-opacity))
• volume = exp(scale_0) * exp(scale_1) * exp(scale_2)

After calling this function, indices[0] will contain the index of the most visible splat, indices[1] the second most visible, and so on.