Fix error amd Update features.

Changed Span<PointData> to ReadOnlySpan<PointData> on method Operate in EffectorBase.cs
Changed the return type of method Operate in EffectorBase.cs from void to PointDate
Added FieldHelper and BlendField function to make the blending work.

Runtime/Cloner/Effector/PlainEffector.cs

@@ -21,21 +21,21 @@ namespace Misaki.ArtTool
         public float3 scale;
         public float uniformScale;
 
-        public override void Operate(int index, float4x4 nodeWorldMatrix, Span<PointData> points)
+        public override PointData Operate(int index, float4x4 nodeWorldMatrix, ReadOnlySpan<PointData> points)
         {
+            var currentPoint = points[index];
             if (!isEnablePosition && !isEnableRotation && !isEnableScale)
             {
-                return;
+                return currentPoint;
             }
 
-            var currentPoint = points[index];
 
             MatrixHelper.DecomposeMatrix(currentPoint.matrix, out var position, out var rotation, out var scale);
 
             var weight = CalculateFieldsWeight(position);
             if (weight == 0)
             {
-                return;
+                return currentPoint;
             }
 
             if (isEnablePosition)
@@ -96,7 +96,7 @@ namespace Misaki.ArtTool
 
             currentPoint.matrix = float4x4.TRS(position, rotation, scale);
 
-            points[index] = currentPoint;
+            return currentPoint;
         }
     }
 }