refactor canvas painter

example/lib/caching_test.dart

@@ -65,9 +65,7 @@ class _CachingTestAppState extends State<CachingTestApp> {
               style: TextStyle(fontSize: 12),
             ),
           ),
-          Expanded(
-            child: LazyCanvas(controller: controller, canvasBackground: const DotGridBackround()),
-          ),
+          Expanded(child: LazyCanvas(controller: controller)),
         ],
       ),
       floatingActionButton: Column(

example/lib/dynamic_widget_example.dart

@@ -116,9 +116,7 @@ class _DynamicWidgetExampleState extends State<DynamicWidgetExample> {
               ],
             ),
           ),
-          Expanded(
-            child: LazyCanvas(controller: controller, canvasBackground: const DotGridBackround()),
-          ),
+          Expanded(child: LazyCanvas(controller: controller)),
         ],
       ),
     );

example/lib/simple_example.dart

@@ -78,7 +78,7 @@ class _SimpleExampleState extends State<SimpleExample> {
       ),
       body: Stack(
         children: [
-          LazyCanvas(controller: controller, canvasBackground: SingleColorBackround(Colors.white)),
+          LazyCanvas(controller: controller),
           Positioned(bottom: 64, left: 16, child: Fps()),
         ],
       ),