Splines: Add resize method to CurveEval

This helps when adding splines to a new curve in parallel.

source/blender/blenkernel/BKE_spline.hh

@@ -535,6 +535,7 @@ struct CurveEval {
   blender::Span<SplinePtr> splines() const;
   blender::MutableSpan<SplinePtr> splines();
 
+  void resize(const int size);
   void add_spline(SplinePtr spline);
   void remove_splines(blender::IndexMask mask);
 

source/blender/blenkernel/intern/curve_eval.cc

@@ -42,6 +42,12 @@ blender::MutableSpan<SplinePtr> CurveEval::splines()
   return splines_;
 }
 
+void CurveEval::resize(const int size)
+{
+  splines_.resize(size);
+  attributes.reallocate(size);
+}
+
 /**
  * \warning Call #reallocate on the spline's attributes after adding all splines.
  */