Update (flipping the default from false -> true) and deprecate Paint.enableDithering. (flutter/engine#44705)

Update: Blocked on https://github.com/flutter/engine/pull/44912 landing,
and merging into google3.

---

Partial work towards https://github.com/flutter/flutter/issues/112498.

_**tl;dr**: In Impeller's backend we intend to _always_ dither
gradients, and never allow any changes long-term (i.e., write your own
shader if you want different behavior) with the assumption that dithered
gradients look better most of the time, and don't typically hurt
elsewhere._

Note that, at the time of this writing, I couldn't find a single case of
this being set explicitly to `true` inside Google, and there are between
[100 and 200 publicly accessible on
GitHub](https://github.com/search?q=%22Paint.enableDithering%22+language%3ADart&type=code&l=Dart),
of which virtually all are setting it explicitly to `true`.

There are some (valid) concerns this would cause a lot of golden-file
image diffs, so I'm going to seek explicit approval from the Google
testing team as well as someone from the framework team before landing
this commit.

engine/src/flutter/flutter_frontend_server/test/to_string_test.dart

@@ -45,7 +45,9 @@ Future<void> main(List<String> args) async {
     ]));
     final ProcessResult runResult = Process.runSync(dart, <String>[regularDill]);
     checkProcessResult(runResult);
-    String paintString = '"Paint.toString":"Paint(Color(0xffffffff))"';
+    // TODO(matanlurey): "dither: true" is now present by default, until it is
+    // remove entirely. See https://github.com/flutter/flutter/issues/112498.
+    String paintString = '"Paint.toString":"Paint(Color(0xffffffff); dither: true)"';
     if (buildDir.contains('release')) {
       paintString = '"Paint.toString":"Instance of \'Paint\'"';
     }

engine/src/flutter/lib/ui/painting.dart

@@ -1487,22 +1487,19 @@ class Paint {
     _data.setInt32(_kDitherOffset, value ? 1 : 0, _kFakeHostEndian);
   }
 
-  /// Whether to dither the output when drawing images.
+  /// Whether to dither the output when drawing some elements such as gradients.
   ///
-  /// If false, the default value, dithering will be enabled when the input
-  /// color depth is higher than the output color depth. For example,
-  /// drawing an RGB8 image onto an RGB565 canvas.
-  ///
-  /// This value also controls dithering of [shader]s, which can make
-  /// gradients appear smoother.
-  ///
-  /// Whether or not dithering affects the output is implementation defined.
-  /// Some implementations may choose to ignore this completely, if they're
-  /// unable to control dithering.
-  ///
-  /// To ensure that dithering is consistently enabled for your entire
-  /// application, set this to true before invoking any drawing related code.
-  static bool enableDithering = false;
+  /// It is not expected that this flag will be used in the future; please leave
+  /// feedback in <https://github.com/flutter/flutter/issues/112498> if there is
+  /// a use case for this flag to remain long term.
+  @Deprecated(
+    'Dithering is now enabled by default on some elements (such as gradients) '
+    'and further support for dithering is expected to be handled by custom '
+    'shaders, so this flag is being removed: '
+    'https://github.com/flutter/flutter/issues/112498.'
+    'This feature was deprecated after 3.14.0-0.1.pre.'
+  )
+  static bool enableDithering = true;
 
   @override
   String toString() {

engine/src/flutter/testing/dart/canvas_test.dart

@@ -203,6 +203,10 @@ void main() {
   }
 
   test('Simple gradient', () async {
+    // TODO(matanl): While deprecated, we still don't want to accidentally
+    // change the behavior of the old API,
+    // https://github.com/flutter/flutter/issues/112498.
+    // ignore: deprecated_member_use
     Paint.enableDithering = false;
     final Image image = await toImage((Canvas canvas) {
       final Paint paint = Paint()..shader = makeGradient();
@@ -217,6 +221,8 @@ void main() {
   }, skip: !Platform.isLinux); // https://github.com/flutter/flutter/issues/53784
 
   test('Simple dithered gradient', () async {
+    // TODO(matanl): Reword this test once we remove the deprecated API.
+    // ignore: deprecated_member_use
     Paint.enableDithering = true;
     final Image image = await toImage((Canvas canvas) {
       final Paint paint = Paint()..shader = makeGradient();