[framework] use ImageFilter for stretch overscroll. (#133613)

Rather than changing the size of the child elements (relaying out text
and potentially changing glyph shape and position due to pixel
snapping), apply the stretch effect as a compositing effect - render the
children to a texture and stretch the texture. If we end up using an
image shader to apply an custom shader we'll need to do this anyway.

Fixes flutter/flutter#129528

### Before


https://github.com/flutter/flutter/assets/8975114/16e9eb57-f864-4093-b4a4-461082b89b43

### After


https://github.com/flutter/flutter/assets/8975114/14cf0167-8922-4876-a325-e3bc154b084f

Author: Jonah Williams

packages/flutter/lib/src/widgets/overscroll_indicator.dart

@@ -786,10 +786,10 @@ class _StretchingOverscrollIndicatorState extends State<StretchingOverscrollIndi
           );
 
           final double viewportDimension = _lastOverscrollNotification?.metrics.viewportDimension ?? mainAxisSize;
-
           final Widget transform = Transform(
             alignment: alignment,
             transform: Matrix4.diagonal3Values(x, y, 1.0),
+            filterQuality: stretch == 0 ? null : FilterQuality.low,
             child: widget.child,
           );
 

packages/flutter/test/widgets/overscroll_stretch_indicator_test.dart

@@ -1124,4 +1124,29 @@ void main() {
 
     await gesture.up();
   });
+
+  testWidgets('Stretch overscroll only uses image filter during stretch effect', (WidgetTester tester) async {
+    final GlobalKey box1Key = GlobalKey();
+    final GlobalKey box2Key = GlobalKey();
+    final GlobalKey box3Key = GlobalKey();
+    final ScrollController controller = ScrollController();
+    await tester.pumpWidget(
+      buildTest(
+        box1Key,
+        box2Key,
+        box3Key,
+        controller,
+        axis: Axis.horizontal,
+      )
+    );
+
+    expect(tester.layers, isNot(contains(isA<ImageFilterLayer>())));
+
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byType(CustomScrollView)));
+    // Overscroll
+    await gesture.moveBy(const Offset(200.0, 0.0));
+    await tester.pumpAndSettle();
+
+    expect(tester.layers, contains(isA<ImageFilterLayer>()));
+  });
 }