compute buffer setup - discard isMainCanvas related logics

Runtime/editor/surface.cs

         public Canvas getCanvas() {
             if (this._canvas == null) {
                 this._canvas = new CommandBufferCanvas(
-                    this._renderTexture, this.devicePixelRatio, this._meshPool, true);
+                    this._renderTexture, this.devicePixelRatio, this._meshPool);
             }
 
             return this._canvas;

Runtime/ui/renderer/cmdbufferCanvas/command_buffer_canvas.cs

     public class CommandBufferCanvas : uiRecorderCanvas {
         readonly PictureFlusher _flusher;
 
-        public CommandBufferCanvas(RenderTexture renderTexture, float devicePixelRatio, MeshPool meshPool, bool isMainCanvas = false)
+        public CommandBufferCanvas(RenderTexture renderTexture, float devicePixelRatio, MeshPool meshPool)
-            this._flusher = new PictureFlusher(renderTexture, devicePixelRatio, meshPool, isMainCanvas);
+            this._flusher = new PictureFlusher(renderTexture, devicePixelRatio, meshPool);
         }
 
         public override float getDevicePixelRatio() {

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

         readonly float _devicePixelRatio;
         readonly MeshPool _meshPool;
 
-        readonly bool _isMainCanvas;
-
         readonly List<RenderLayer> _layers = new List<RenderLayer>();
         RenderLayer _currentLayer;
         uiRect? _lastScissor;
             }
         }
 
-        public PictureFlusher(RenderTexture renderTexture, float devicePixelRatio, MeshPool meshPool, bool isMainCanvas) {
+        public PictureFlusher(RenderTexture renderTexture, float devicePixelRatio, MeshPool meshPool) {
             D.assert(renderTexture);
             D.assert(devicePixelRatio > 0);
             D.assert(meshPool != null);
             this._devicePixelRatio = devicePixelRatio;
             this._meshPool = meshPool;
-            this._isMainCanvas = isMainCanvas;
 
             this.___drawTextDrawMeshCallback = this._drawTextDrawMeshCallback;
             this.___drawPathDrawMeshCallback2 = this._drawPathDrawMeshCallback2;

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

             var renderTexture = new RenderTexture(desc);
             renderTexture.hideFlags = HideFlags.HideAndDontSave;
 
-            var canvas = new CommandBufferCanvas(renderTexture, devicePixelRatio, meshPool, false);
+            var canvas = new CommandBufferCanvas(renderTexture, devicePixelRatio, meshPool);
             canvas.translate(-bounds.left, -bounds.top);
             canvas.concat(transform);
             canvas.drawPicture(picture);