Update (flipping the default from false -> true) and deprecate Paint.enableDithering.

[matanlurey]

Update: Blocked on #44912 landing, and merging into google3.

---

Partial work towards flutter/flutter#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, 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.

[matanlurey]

It appears this will break at least 1 framework test:

flutter/test/painting/text_style_test.dart: backgroundColor [E]                                                                                   
  Expected: contains 'background: Paint(Color(0xff00ff00))'
    Actual: 'TextStyle(color: unspecified, decoration: unspecified, decorationColor: unspecified, decorationStyle: unspecified, decorationThickness: unspecified, fontWeight: unspecified, fontStyle: unspecified, textBaseline: unspecified, fontFamily: unspecified, fontFamilyFallback: unspecified, fontSize: unspecified, letterSpacing: unspecified, wordSpacing: unspecified, height: unspecified, leadingDistribution: unspecified, locale: unspecified, background: Paint(Color(0xff00ff00); dither: true), foreground: unspecified, shadows: unspecified, fontFeatures: unspecified, fontVariations: unspecified)'
     Which: does not contain 'background: Paint(Color(0xff00ff00))'
  
  package:matcher                                     expect
  package:flutter_test/src/widget_tester.dart 458:18  expect
  test/painting/text_style_test.dart 462:5            main.<fn>

... and a number of engine tests will need to be updated. One example:

flutter/shell/platform/embedder/tests/embedder_gl_unittests.cc:1469: Failure
Value of: ImageMatchesFixture( FixtureNameForBackend(backend, "gradient.png"), rendered_scene)
  Actual: false
Expected: true
�[0;31m[  FAILED  ] �[mEmbedderTestGlVk/EmbedderTestMultiBackend.CanRenderGradientWithCompositorOnNonRootLayer/1, where GetParam() = 4-byte object <03-00 00-00> (15612 ms)
�[0;32m[----------] �[m1 test from EmbedderTestGlVk/EmbedderTestMultiBackend (15613 ms total)

�[0;32m[----------] �[mGlobal test environment tear-down
�[0;32m[==========] �[m1 test from 1 test suite ran. (15614 ms total)
�[0;32m[  PASSED  ] �[m0 tests.
�[0;31m[  FAILED  ] �[m1 test, listed below:
�[0;31m[  FAILED  ] �[mEmbedderTestGlVk/EmbedderTestMultiBackend.CanRenderGradientWithCompositorOnNonRootLayer/1, where GetParam() = 4-byte object <03-00 00-00>

A couple of options:

1. I could hardcode this off in a test environment(s)
2. I could update the tests to be less brittle, i.e. allow dithering/check less pixels
3. I could update the tests to always assume dithering

Thoughts?

[chinmaygarde]

This will definitely blow up Scubas. But if we think this is the right change, we shouldn't let the pain of updating Scubas stop us. @CaseyHillers indicated that he has a workflow for dealing with a lot of Scubas. And it makes sense to exercise that playbook on a non-on-fire change.

[CaseyHillers]

Preferably, can we update this outside of the roll process? @LongCatIsLooong recently did this with a build flag, and was able to update all the scubas without blocking the roll.

[matanlurey]

@chinmaygarde:

This will definitely blow up Scubas

After disabling Skia image diffing, I'm actually not sure it will anymore. There are virtually no gradients internally.

@CaseyHillers:

Preferably, can we update this outside of the roll process?

Yes. I'm going to make sure google3 looks green (or at least, 99% green) before I land this.

[chinmaygarde]

Looks like there was a blocker that is progressing. Just wanted to unblock this.

[matanlurey]

Blocked by flutter/flutter#133375.