fix the case where antiAliasing is 0 (disabled).

Runtime/ui/painting/canvas_impl.cs

 
         void _drawLayer(RenderLayer layer, CommandBuffer cmdBuf) {
             foreach (var subLayer in layer.layers) {
-                cmdBuf.GetTemporaryRT(subLayer.rtID, new RenderTextureDescriptor(
+                var desc = new RenderTextureDescriptor(
-                    msaaSamples = QualitySettings.antiAliasing,
-                }, FilterMode.Bilinear);
+                };
+                
+                if (QualitySettings.antiAliasing != 0) {
+                    desc.msaaSamples = QualitySettings.antiAliasing;
+                }
+                
+                cmdBuf.GetTemporaryRT(subLayer.rtID, desc, FilterMode.Bilinear);
                 this._drawLayer(subLayer, cmdBuf);
             }
 

Tests/Editor/CanvasAndLayers.cs

                     width,
                     height,
                     RenderTextureFormat.Default, 24) {
-                    msaaSamples = QualitySettings.antiAliasing,
+                if (QualitySettings.antiAliasing != 0) {
+                    desc.msaaSamples = QualitySettings.antiAliasing;
+                }
+                
                 this._renderTexture = RenderTexture.GetTemporary(desc);
             }
         }