Fix renderSegment reversed range logic and align rounding behavior.

[ej-sanmartin]

Fixes #2238

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This PR fixes two issues in BezierCurve::renderSegment:

• Reversed argument handling (start > end)
  The existing else if (end > start) branch was unreachable due to the
  preceding start < end check. As a result, calling renderSegment with
  reversed parameters returned the full curve instead of the intended subrange.
  The condition is corrected so reversed ranges now slice the proper segment.

• Asymmetric rounding when slicing sampled vertices
  The forward and reversed paths previously used different rounding rules,
  causing renderSegment(a, b) and renderSegment(b, a) to select slightly
  different vertex ranges. Index calculation is now normalized so both cases
  refer to the same portion of the sampled curve.

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Result

renderSegment now handles reversed arguments consistently and returns the
correct sub-curve in both orders, with stable and predictable indexing.