Filters a raw trail by removing points too close to the previous kept point.
A point is kept only if its Euclidean distance from the last kept point exceeds threshold.
The first point is always kept.
Returns the opacity [0.0, 1.0] for a trail point at a given playback position.
The point is fully opaque at point_time_ms and fades to 0.0 after fade_duration_ms.
Returns 0.0 if the point has not yet been reached or has fully faded.