Fix text field selection toolbar under Opacity (#31097)

## Description

As we've introduced offset to the Opacity layer, we have to override
`applyTransform` to make Leader/FollowerLayer work correctly.

## Related Issues

Fixes https://github.com/flutter/flutter/issues/30587

Together with https://github.com/flutter/engine/pull/8585,
this test will also exercise test against
https://github.com/flutter/flutter/issues/30586.

## Tests

I added the following tests:
* text field selection toolbar renders correctly inside opacity

packages/flutter/lib/src/rendering/layer.dart

@@ -1282,6 +1282,13 @@ class OpacityLayer extends ContainerLayer {
     }
   }
 
+  @override
+  void applyTransform(Layer child, Matrix4 transform) {
+    assert(child != null);
+    assert(transform != null);
+    transform.translate(offset.dx, offset.dy);
+  }
+
   @override
   ui.EngineLayer addToScene(ui.SceneBuilder builder, [ Offset layerOffset = Offset.zero ]) {
     bool enabled = firstChild != null;  // don't add this layer if there's no child

packages/flutter/test/material/text_field_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:io';
 import 'dart:math' as math;
 import 'dart:ui' as ui show window;
 
@@ -380,6 +381,58 @@ void main() {
     expect(textField.cursorRadius, const Radius.circular(3.0));
   });
 
+  testWidgets('text field selection toolbar renders correctly inside opacity', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Center(
+            child: Container(
+              width: 100,
+              height: 100,
+              child: const Opacity(
+                opacity: 0.5,
+                child: TextField(
+                  decoration: InputDecoration(hintText: 'Placeholder'),
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.showKeyboard(find.byType(TextField));
+
+    const String testValue = 'A B C';
+    tester.testTextInput.updateEditingValue(
+        const TextEditingValue(
+          text: testValue
+        )
+    );
+    await tester.pump();
+
+    // The selectWordsInRange with SelectionChangedCause.tap seems to be needed to show the toolbar.
+    // (This is true even if we provide selection parameter to the TextEditingValue above.)
+    final EditableTextState state = tester.state<EditableTextState>(find.byType(EditableText));
+    state.renderEditable.selectWordsInRange(from: const Offset(0, 0), cause: SelectionChangedCause.tap);
+
+    expect(state.showToolbar(), true);
+
+    // This is needed for the AnimatedOpacity to turn from 0 to 1 so the toolbar is visible.
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    // Sanity check that the toolbar widget exists.
+    expect(find.text('PASTE'), findsOneWidget);
+
+    await expectLater(
+      // The toolbar exists in the Overlay above the MaterialApp.
+      find.byType(Overlay),
+      matchesGoldenFile('text_field_opacity_test.0.0.png'),
+      skip: !Platform.isLinux,
+    );
+  });
+
   // TODO(hansmuller): restore these tests after the fix for #24876 has landed.
   /*
   testWidgets('cursor layout has correct width', (WidgetTester tester) async {