Matan Lurey bc3b2e01ef [Impeller] Implement, non user-facing, dithering for LinearGradients. (flutter/engine#44181) ...

Partial work towards https://github.com/flutter/flutter/issues/131450.

---

Run the Playground locally:

```bash
$ENGINE/out/host_debug_unopt/impeller_unittests \
  --enable_playground \
  --gtest_filter="*CanRenderLinearGradientWithDithering*"
```

Summary of changes:

- Added a playground/test for dithering disabled and enabled.
- Added `bool dither` to Impeller's `Paint`, but `SetDither` (use-facing) is ignored.
- Converted [Skia's dithering](f9de059517/src/opts/SkRasterPipeline_opts.h (L1717)) to GLSL and invoked it when `dither` is set.

## Before

![Screenshot 2023-07-31 at 1 29 16 PM](https://github.com/flutter/engine/assets/168174/51d2f7a0-dc22-44fe-b7f9-990b826c5fd9)

## After

![Screenshot 2023-07-31 at 1 29 25 PM](https://github.com/flutter/engine/assets/168174/78da2efe-2c5d-438f-b7f7-d8eb092c6a2c)

## Deleted Scenes

<details>
<summary>Here are some of my earlier attempts that are fun to share :)</summary>

![Screenshot 2023-07-31 at 11 35 07 AM](https://github.com/flutter/engine/assets/168174/719f97fc-1a3d-4920-8687-486c4de28b79)

![Screenshot 2023-07-31 at 12 12 38 PM](https://github.com/flutter/engine/assets/168174/ed262f83-442f-484b-8288-30e8e0d5e768)

![Screenshot 2023-07-31 at 12 19 56 PM](https://github.com/flutter/engine/assets/168174/057b1f95-fbd2-4ae2-bb25-8dd967cf8466)

![Screenshot 2023-07-31 at 12 25 59 PM](https://github.com/flutter/engine/assets/168174/34857c6e-49cd-40c1-9e91-646b7bfbf97c)

![Screenshot 2023-07-31 at 12 44 08 PM](https://github.com/flutter/engine/assets/168174/3b272428-b5be-4ca5-8cfe-1b12062a64f4)

</details>

2023-08-01 02:02:25 +00:00

..

src/flutter

[Impeller] Implement, non user-facing, dithering for LinearGradients. (flutter/engine#44181)

2023-08-01 02:02:25 +00:00