Refactor visibility_detector to avoid forcing compositing.

Instead, use methods on RenderObject and new framework API to
listen to compositing and determine the transform/clip.

Removes an O(N^2) algorithm from the ancestor walk.

Deletes the custom layer that is no longer used.

Minor update to main.dart to be more desktop friendly.

Tests adjusted to use new API. Removed API has no usages
in internal codesearch.

packages/visibility_detector/example/lib/main.dart

@@ -74,6 +74,7 @@ class VisibilityDetectorDemo extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialApp(
       title: title,
+      scrollBehavior: const MaterialScrollBehavior().copyWith(scrollbars: false),
       theme: ThemeData(primarySwatch: Colors.blue),
       home: VisibilityDetectorDemoPage(key: key, useSlivers: useSlivers),
     );