[Impeller] Fix division by zero for transparent shadows

[bdero]

A division by zero happens if the shadow color is fully transparent, and the NaNs are getting assigned to the RGB values of the paint color being handed to the rrect shadow draw. This doesn't cause a problem when wide gamut is off because NaN output ends up being interpreted as zero (thus making the shadow output fully transparent black, which happens to be the expected and correct result). However, when a wide gamut attachment is present, the NaN output ends up being interpreted as a negative value.

Reproduction app:

import 'package:flutter/material.dart';

void main() => runApp(const GeneralDialogApp());

class EvilPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final Rect rect = Offset.zero & size;
    canvas.drawPaint(Paint()..color = Colors.white);
    canvas.saveLayer(null, Paint()..blendMode = BlendMode.srcOver);
    canvas.drawShadow(Path()..addRect(Rect.fromLTRB(100, 100, 300, 300)),
        Colors.black54, 15, false);
    canvas.drawShadow(Path()..addRect(Rect.fromLTRB(100, 100, 300, 300)),
        Colors.black54, 15, false);
    canvas.drawShadow(Path()..addRect(Rect.fromLTRB(100, 100, 300, 300)),
        Colors.transparent, 15, false);
    canvas.restore();
  }

  @override
  bool shouldRepaint(EvilPainter oldDelegate) => false;
  @override
  bool shouldRebuildSemantics(EvilPainter oldDelegate) => false;
}

class GeneralDialogApp extends StatelessWidget {
  const GeneralDialogApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      restorationScopeId: 'app',
      home: CustomPaint(painter: EvilPainter()),
    );
  }
}

Before:

After:

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Changes reported for pull request #41391 at sha 20c8453

[zanderso]

Nice find!

[zanderso]

Skia gold seems confused. I'm going to go ahead and land this.