`next_gen_ui_demo`: Add `dispose` handler for `Picture`s (#1889)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

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<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

next_gen_ui_demo/lib/common/shader_effect.dart

@@ -283,10 +283,10 @@ class _RenderShaderSamplerBuilderWidget extends RenderProxyBox {
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (size.isEmpty || !_enabled) {
+    if (size.isEmpty) {
       return;
     }
-    assert(offset == Offset.zero);
+    assert(!_enabled || offset == Offset.zero);
     return super.paint(context, offset);
   }
 }
@@ -359,4 +359,10 @@ class _ShaderSamplerBuilderLayer extends OffsetLayer {
     _lastPicture = pictureRecorder.endRecording();
     builder.addPicture(offset, _lastPicture!);
   }
+
+  @override
+  void dispose() {
+    _lastPicture?.dispose();
+    super.dispose();
+  }
 }

next_gen_ui_demo/pubspec.yaml

@@ -4,7 +4,7 @@ publish_to: 'none'
 version: 0.1.0
 
 environment:
-  sdk: '>=3.0.0-417.4.beta <4.0.0'
+  sdk: ^3.0.0
 
 dependencies:
   cupertino_icons: ^1.0.5