Add convex path and non-AA paint to shader warm-up (flutter#28614)

One of our important client's SKP shows that this could improve one of
their janky frame by 20ms. This also improves our
flutter_gallery__transition_perf's worst frame time by ~20ms.

On the other hand, 15ms has been added to the start-up latency. I guess
it's a little faster to compile the shader on the start-up because we're
compiling a lot of shaders there and the CPU cache must be hot.

## Related Issues

flutter#813

packages/flutter/lib/src/painting/binding.dart

@@ -38,8 +38,8 @@ mixin PaintingBinding on BindingBase, ServicesBinding {
   /// shaders that are not covered by [DefaultShaderWarmUp], it may cause jank
   /// in the middle of an animation or interaction. In that case, set
   /// [shaderWarmUp] to a custom [ShaderWarmUp] before calling [initInstances]
-  /// (usually before [runApp] for normal flutter apps, and before
-  /// [enableFlutterDriverExtension] for flutter drive tests). Paint the scene
+  /// (usually before [runApp] for normal Flutter apps, and before
+  /// [enableFlutterDriverExtension] for Flutter drive tests). Paint the scene
   /// in the custom [ShaderWarmUp] so Flutter can pre-compile and cache the
   /// shaders during startup. The warm up is only costly (100ms-200ms,
   /// depending on the shaders to compile) during the first run after the
@@ -49,6 +49,10 @@ mixin PaintingBinding on BindingBase, ServicesBinding {
   /// Currently the warm-up happens synchronously on the GPU thread which means
   /// the rendering of the first frame on the GPU thread will be postponed until
   /// the warm-up is finished.
+  ///
+  /// See also:
+  ///
+  ///  * [ShaderWarmUp], the interface of how this warm up works.
   static ShaderWarmUp shaderWarmUp = const DefaultShaderWarmUp();
 
   /// The singleton that implements the Flutter framework's image cache.

packages/flutter/lib/src/painting/shader_warm_up.dart

@@ -9,26 +9,45 @@ import 'package:flutter/foundation.dart';
 
 /// Interface for drawing an image to warm up Skia shader compilations.
 ///
-/// When Skia first sees a certain type of draw operations on GPU, it needs to
-/// compile the corresponding shader. The compilation can be slow (20ms-200ms).
-/// Having that time as a startup latency is often better than having a jank in
-/// the middle of an animation.
+/// When Skia first sees a certain type of draw operation on the GPU, it needs
+/// to compile the corresponding shader. The compilation can be slow (20ms-
+/// 200ms). Having that time as startup latency is often better than having
+/// jank in the middle of an animation.
 ///
 /// Therefore, we use this during the [PaintingBinding.initInstances] call to
 /// move common shader compilations from animation time to startup time. By
-/// default, a [DefaultShaderWarmUp] is used. Create a custom [ShaderWarmUp]
-/// subclass to replace [PaintingBinding.shaderWarmUp] before
-/// [PaintingBinding.initInstances] is called. Usually, that can be done before
-/// calling [runApp].
+/// default, a [DefaultShaderWarmUp] is used. If needed, app developers can
+/// create a custom [ShaderWarmUp] subclass and hand it to
+/// [PaintingBinding.shaderWarmUp] (so it replaces [DefaultShaderWarmUp])
+/// before [PaintingBinding.initInstances] is called. Usually, that can be
+/// done before calling [runApp].
 ///
-/// This warm up needs to be run on each individual device because the shader
+/// To determine whether a draw operation is useful for warming up shaders,
+/// check the difference in the `worst_frame_rasterizer_time_millis` benchmarks.
+/// Also, tracing with `flutter run --profile --trace-skia` may reveal whether
+/// there is shader-compilation-related jank. If there is such jank, some long
+/// `GrGLProgramBuilder::finalize` calls would appear in the middle of an
+/// animation. Their parent calls, which look like `XyzOp` (e.g., `FillRecOp`,
+/// `CircularRRectOp`) would suggest Xyz draw operations are causing the shaders
+/// to be compiled. A useful shader warm-up draw operation would eliminate such
+/// long compilation calls in the animation. To double-check the warm-up, trace
+/// with `flutter run --profile --trace-skia --start-paused`. The
+/// `GrGLProgramBuilder` with the associated `XyzOp` should appear during
+/// startup rather than in the middle of a later animation.
+///
+/// This warm-up needs to be run on each individual device because the shader
 /// compilation depends on the specific GPU hardware and driver a device has. It
 /// can't be pre-computed during the Flutter engine compilation as the engine is
-/// device agnostic.
+/// device-agnostic.
 ///
-/// If no warm up is desired (e.g., when the startup latency is crucial), set
+/// If no warm-up is desired (e.g., when the startup latency is crucial), set
 /// [PaintingBinding.shaderWarmUp] either to a custom ShaderWarmUp with an empty
 /// [warmUpOnCanvas] or null.
+///
+/// See also:
+///
+///  * [PaintingBinding.shaderWarmUp], the actual instance of [ShaderWarmUp]
+///    that's used to warm up the shaders.
 abstract class ShaderWarmUp {
   /// Allow const constructors for subclasses.
   const ShaderWarmUp();
@@ -101,12 +120,27 @@ class DefaultShaderWarmUp extends ShaderWarmUp {
     path.moveTo(60.0, 20.0);
     path.quadraticBezierTo(60.0, 60.0, 20.0, 60.0);
 
-    final List<ui.Path> paths = <ui.Path>[rrectPath, circlePath, path];
+    final ui.Path convexPath = ui.Path();
+    convexPath.moveTo(20.0, 30.0);
+    convexPath.lineTo(40.0, 20.0);
+    convexPath.lineTo(60.0, 30.0);
+    convexPath.lineTo(60.0, 60.0);
+    convexPath.lineTo(20.0, 60.0);
+    convexPath.close();
+
+    // Skia uses different shaders based on the kinds of paths being drawn and
+    // the associated paint configurations. According to our experience and
+    // tracing, drawing the following paths/paints generates various of
+    // shaders that are commonly used.
+    final List<ui.Path> paths = <ui.Path>[rrectPath, circlePath, path, convexPath];
 
     final List<ui.Paint> paints = <ui.Paint>[
       ui.Paint()
         ..isAntiAlias = true
         ..style = ui.PaintingStyle.fill,
+      ui.Paint()
+        ..isAntiAlias = false
+        ..style = ui.PaintingStyle.fill,
       ui.Paint()
         ..isAntiAlias = true
         ..style = ui.PaintingStyle.stroke