Silenced warning about onCanvasGUI (#15)

* Silenced warning about onCanvasGUI

* Converted onCanvasGUI to an event

* @peeweek's pointed out this should be an event, so doing it this way rather than just silencing the warning

Editor/Common/Canvas/VFXToolboxCanvas.cs

+using System;
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
         }
         private Texture m_Texture;
 
-        internal CanvasGUIDelegate onCanvasGUI;
-        internal delegate void CanvasGUIDelegate();
+        internal event EventHandler onCanvasGUI;
 
         internal VFXToolboxCanvas(Rect displayRect, string shaderName = "Packages/com.unity.vfx-toolbox/Editor/Common/Canvas/Shaders/VFXToolboxCanvas.shader") 
         {
 #endif
 
                 if (showGrid) DrawGrid();
-                if (onCanvasGUI != null)
-                    onCanvasGUI();
+                onCanvasGUI?.Invoke(this, EventArgs.Empty);
             }
             else
                 GUI.Label(LocalRect, VFXToolboxGUIUtility.Get("No Texture"), EditorStyles.centeredGreyMiniLabel);