Splines: Convenience methods for point offsets of multiple splines

Since spline data is stored separately for each spline, the data often
needs to be flattened into a separate array. It's helpful to have the
necessarily-sequential part of that split off into a separate method.
I've found myself using functions like these in quite a few places.

source/blender/blenkernel/BKE_spline.hh

@@ -493,6 +493,9 @@ class CurveEval {
   void translate(const blender::float3 &translation);
   void transform(const blender::float4x4 &matrix);
   void bounds_min_max(blender::float3 &min, blender::float3 &max, const bool use_evaluated) const;
+
+  blender::Array<int> control_point_offsets() const;
+  blender::Array<int> evaluated_point_offsets() const;
 };
 
 std::unique_ptr<CurveEval> curve_eval_from_dna_curve(const Curve &curve);

source/blender/blenkernel/intern/curve_eval.cc

@@ -23,6 +23,7 @@
 #include "BKE_curve.h"
 #include "BKE_spline.hh"
 
+using blender::Array;
 using blender::float3;
 using blender::float4x4;
 using blender::Span;
@@ -82,6 +83,40 @@ void CurveEval::bounds_min_max(float3 &min, float3 &max, const bool use_evaluate
   }
 }
 
+/**
+ * Return the start indices for each of the curve spline's evaluated points, as if they were part
+ * of a flattened array. This can be used to facilitate parallelism by avoiding the need to
+ * accumulate an offset while doing more complex calculations.
+ *
+ * \note The result array is one longer than the spline count; the last element is the total size.
+ */
+blender::Array<int> CurveEval::control_point_offsets() const
+{
+  Array<int> offsets(splines_.size() + 1);
+  int offset = 0;
+  for (const int i : splines_.index_range()) {
+    offsets[i] = offset;
+    offset += splines_[i]->size();
+  }
+  offsets.last() = offset;
+  return offsets;
+}
+
+/**
+ * Exactly like #control_point_offsets, but uses the number of evaluated points instead.
+ */
+blender::Array<int> CurveEval::evaluated_point_offsets() const
+{
+  Array<int> offsets(splines_.size() + 1);
+  int offset = 0;
+  for (const int i : splines_.index_range()) {
+    offsets[i] = offset;
+    offset += splines_[i]->evaluated_points_size();
+  }
+  offsets.last() = offset;
+  return offsets;
+}
+
 static BezierSpline::HandleType handle_type_from_dna_bezt(const eBezTriple_Handle dna_handle_type)
 {
   switch (dna_handle_type) {