[CP-beta]Eliminate some early returns in uber_sdf.frag to fix broken UberSDF AA on Windows - #190378

…A on Windows (flutter#190260)

See
flutter#189827 (comment)
for some more context. That comment shows 2 issues:
1. UberSDF AA is broken on Windows
2. Tessellated AA is broken on OpenGLES

Fixes
flutter#189827 (comment)

This PR fixes the first issue. The second issue is tracked at
flutter#190060.

This PR flattens some branching in uber_sdf.frag to eliminate some early
returns.

Something about the dFdx and dFdy derivatives are incompatible with
ANGLE and/or D3D, making uberSDF antialiasing fail on Windows. I don't
know what the exact limits are of when dFdx and dFdy are allowed. From
asking Gemini, it may be related to branching, non-uniform branching,
loops, or early return statements. After some manual testing,
eliminating early return statements in functions which use dFdx and dFdy
(either directly, or indirectly through a child call) seems sufficient
to fix this.

### Demo app

<details> <summary>Demo:</summary>

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @OverRide
  Widget build(BuildContext context) {
    return InteractiveViewer(
      maxScale: 10,
      child: Container(
        color: Colors.black,
        child: CustomPaint(painter: TestPainter()),
      ),
    );
  }
}

class TestPainter extends CustomPainter {
  @OverRide
  void paint(Canvas canvas, Size size) {
    var filledPaint = Paint()..color = Colors.white;
    var strokedPaint = Paint()
      ..color = Colors.white
      ..style = PaintingStyle.stroke
      ..strokeWidth = 5;
    const rect = Rect.fromLTWH(20, 20, 30.5, 15.5);

    // circle
    canvas.drawCircle(Offset(35, 25), 15.5, filledPaint);
    canvas.save();
    canvas.translate(50, 0);
    canvas.drawCircle(Offset(35, 25), 15.5, strokedPaint);
    canvas.restore();

    // oval
    canvas.translate(0, 50);
    canvas.drawOval(rect, filledPaint);
    canvas.save();
    canvas.translate(50, 0);
    canvas.drawOval(rect, strokedPaint);
    canvas.restore();

    // rounded rect
    canvas.translate(0, 50);
    canvas.drawRRect(RRect.fromRectAndRadius(rect, .circular(5)), filledPaint);
    canvas.save();
    canvas.translate(50, 0);
    canvas.drawRRect(
      RRect.fromRectAndRadius(rect, .circular(5)),
      strokedPaint,
    );
    canvas.restore();

    // column 2
    canvas.translate(120, -150);

    // rect
    canvas.translate(0, 50);
    canvas.drawRect(rect, filledPaint);
    canvas.save();
    canvas.translate(50, 0);
    canvas.drawRect(rect, strokedPaint);
    canvas.restore();

    // rotated rect
    canvas.translate(0, 50);
    canvas.save();
    canvas.rotate(0.3);
    canvas.drawRect(rect, filledPaint);
    canvas.restore();
    canvas.save();
    canvas.translate(50, 0);
    canvas.rotate(0.3);
    canvas.drawRect(rect, strokedPaint);
    canvas.restore();

    // RSuperellipse
    canvas.translate(0, 50);
    canvas.drawRSuperellipse(
      RSuperellipse.fromRectAndRadius(rect, .circular(5)),
      filledPaint,
    );
    canvas.save();
    canvas.translate(50, 0);
    canvas.drawRSuperellipse(
      RSuperellipse.fromRectAndRadius(rect, .circular(5)),
      strokedPaint,
    );
    canvas.restore();
  }

  @OverRide
  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}

```

</details>

### Before:

<img width="232" height="189" alt="image"
src="https://github.com/user-attachments/assets/ef75b0ea-62be-49a6-9c20-1507ed9e1435"
/>

500% zoom:
<img width="1150" height="938" alt="image"
src="https://github.com/user-attachments/assets/e199cbf1-222c-4e1d-b250-e5649b212e86"
/>


### After:

<img width="232" height="189" alt="image"
src="https://github.com/user-attachments/assets/181c9316-e3f1-4b10-b474-fdc2baf74e50"
/>

500% zoom:
<img width="1150" height="938" alt="image"
src="https://github.com/user-attachments/assets/26783e17-d5de-46f3-a0af-9b8801aeb122"
/>

Notice that stroked RSuperellipse does not have AA. This confused me for
a long time. Eventually I figured out that stroked RSuperellipse is
[always drawn with a
path](https://github.com/flutter/flutter/blob/b1ae9ca144127fa87eb86d0cdf73f6c028964ec3/engine/src/flutter/display_list/dl_builder.cc#L1371-L1378).
So it does not go through the UberSDF code. Paths render with
tessellation, which is supposed to use MSAA but is currently not
functioning properly on Windows.


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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

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---------

Co-authored-by: gaaclarke <[email protected]>