Support for TwoPointConical gradients

[dnfield]

Engine side fix for flutter/flutter#17106

This patch updates Gradient.radial to support an optional focal Offset and radius. If these are provided, it the call will instantiate an SkTwoPointConicalGradient instead of plain old SkRadialGradient.

Skia fiddle here: https://fiddle.skia.org/c/adc33d57c035fcb8bcbd4c3ad6381c13
Interesting/relevant reading for Skia end: https://skia.org/dev/design/conical

I initially wanted to name this something like Gradient.conical (similar to Skia's naming convention), but I'm afraid that will cause confusion for a lot of users who google "conical gradients" and find stuff about planned support for what Skia calls Sweep gradients under that name (in fact, I got a bit muddled myself before realizing that the terms are not being used identically by Skia and the proposed CSS standard/top Google search results). The tiling mode documentation should not need to be changed for this.

My primary use case for this is to enable better support of SVG's <radialGradient> element, but this is not SVG specific.

There'd be a similar update in Flutter SDK to support this in ui.Gradient.radial over there.

[chinmaygarde]

cc @Hixie for comments on Dart code.

[Hixie]

oh man, we should have used named arguments. this has become sad.

[dnfield]

Yeah :( but that'd be a a breaking change. I suppose it could be another method and deprecate this one....

[Hixie]

we could do another method, but as you point out in the commit message, that's got its own problems.

[Hixie]

let's just do a good job of the framework wrapper, then this particular method won't matter much.

[Hixie]

[aam]

This has broken flutter tests(once this lands in flutter/flutter).

╰─➤  ../../bin/flutter  --local-engine host_debug_unopt test test/painting/gradient_test.dart
00:05 +0: loading $FH/flutter/packages/flutter/test/painting/gradient_test.dart                     Shell: Do not know weight for: TeXGyreSchola (BoldItalic) 
00:06 +5 -1: RadialGradient with AlignmentDirectional [E]                                                                                  
  Expected: return normally
    Actual: <Closure: () => Shader>
     Which: threw ?:<'dart:ui/painting.dart': Failed assertion: line 2330 pos 14: 'center != Offset.zero || focal != Offset.zero': is not true.>
  
  package:test                                        expect
  package:flutter_test/src/widget_tester.dart 147:16  expect
  test/painting/gradient_test.dart 176:5              main.<fn>
  
00:06 +10 -1: Some tests failed.   

[dnfield]

Urfh. Will fix but looks like it should be reverted til then. The assertion should also pass if focal is null.