Previously, when the start input was greater than the end input,
the spline was resized to a single point. That is correct, but the
single point wasn't placed properly along the spline. Now, it is
placed according to the "Start" value, as if the trim started, but
couldn't continue because the "End" value was smaller.
This behavior is handled with a separate code path to keep each
simpler and avoid special cases. Any cleanup to reduce duplication
should focus on making each code path shorter separately rather
than merging them.
Also included are some changes to `lookup_control_point_position`
to support cyclic splines, though this single-point method is still
disabled on cyclic splines for consistency.
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