add configs for user to disable some features in their app

usecase: in Editor app, the RasterCache may should be disabled to avoid Engine errors when the RenderTexture is too big

Runtime/ui/renderer/cmdbufferCanvas/rendering/canvas_shader_initializer.cs

+using Unity.UIWidgets.editor;
-        const bool enableComputeBuffer = false;
-
         const bool enableDebugLog = false;
 
         public static bool supportComputeBuffer;
         }
 
         static void InitShaders() {
-            supportComputeBuffer = enableComputeBuffer && SystemInfo.supportsComputeShaders && IsShaderSupported();
+            supportComputeBuffer = !WindowConfig.disableComputeBuffer && SystemInfo.supportsComputeShaders && IsShaderSupported();
 
             if (!supportComputeBuffer) {
                 DebugAssert(false, "init default shaders");

Runtime/ui/renderer/compositeCanvas/flow/raster_cache.cs

                 return false;
             }
 
+            if (Window.instance.windowConfig.disableRasterCache) {
+                return false;
+            }
+            
             var bounds = picture.paintBounds;
             if (bounds.isEmpty) {
                 return false;
             }
 
             if (picture.isDynamic) {
+                return false;
+            }
+
+            //https://forum.unity.com/threads/rendertexture-create-failed-rendertexture-too-big.58667/
+            if (picture.paintBounds.size.width > 4096 ||
+                picture.paintBounds.size.height > 4096) {
                 return false;
             }
 

Runtime/ui/window.cs

 using System;
 using System.Collections.Generic;
 using Unity.UIWidgets.async;
+using Unity.UIWidgets.editor;
 using Unity.UIWidgets.foundation;
 using UnityEngine;
 
 
         protected int _antiAliasing = defaultAntiAliasing;
 
-
+
+        public WindowConfig windowConfig = WindowConfig.defaultConfig;
 
         protected Size _physicalSize = Size.zero;
 

Samples/UIWidgetsGallery/GalleryMain.cs

 using UIWidgetsGallery.gallery;
 using Unity.UIWidgets.engine;
-using Unity.UIWidgets.material;
 using Unity.UIWidgets.ui;
 using Unity.UIWidgets.widgets;
 using UnityEngine;

Runtime/editor/window_config.cs

+using Unity.UIWidgets.foundation;
+
+namespace Unity.UIWidgets.editor {
+    public class WindowConfig {
+        public readonly bool disableRasterCache;
+
+        static bool? _disableComputeBuffer = null;
+
+        public static bool disableComputeBuffer {
+            //disable compute buffer by default for now
+            get { return _disableComputeBuffer ?? true; }
+            set {
+                D.assert(_disableComputeBuffer == null
+                    || _disableComputeBuffer == value
+                    , () => "The global settings of [disableComputeBuffer] cannot be initiated for multiple times!");
+
+                _disableComputeBuffer = value;
+            }
+        }
+
+        public WindowConfig(bool disableRasterCache) {
+            this.disableRasterCache = disableRasterCache;
+        }
+
+        public static readonly WindowConfig defaultConfig = new WindowConfig(
+            disableRasterCache: false
+        );
+    }
+}