feat: add a new public property 'useStencil'

Packages/SoftMaskForUGUI/Scripts/SoftMaskable.cs

@@ -71,6 +71,20 @@ namespace Coffee.UISoftMask
             set { m_RaycastFilter = value; }
         }
 
+        /// <summary>
+        /// Use stencil to mask.
+        /// </summary>
+        public bool useStencil
+        {
+            get { return m_UseStencil; }
+            set
+            {
+                if (m_UseStencil == value) return;
+                m_UseStencil = value;
+                graphic.SetMaterialDirtyEx();
+            }
+        }
+
         /// <summary>
         /// The graphic associated with the soft mask.
         /// </summary>