Rust PNG decoder

This directory contains experimental code for

• Providing SkCodec API for decoding PNG images using Rust png crate.
• Providing SkEncoder API for encoding PNG images using Rust png crate.

See the following document for more details: https://docs.google.com/document/d/1glx5ue5JDlCld5WzWgTOGK3wsMErQFnkY5N5Dsbi91Y/edit?usp=sharing

Chromium build instructions

To build and test the code from this directory from Chromium:

1. gn args out/... and set enable_rust_png = true
2. autoninja -C out/... gfx_unittests blink_platform_unittests chrome
3. out/.../gfx_unittests --gtest_filter=RustEnabled*
4. out/.../blink_platform_unittests --gtest_filter=RustEnabled*
5. out/.../chrome --enable-features=RustyPng

Skia build instructions

Bazel

To build the code from this directory from Skia (testing via Bazel is not supported yet):

$ cd skia-repo-root
$ bazelisk build experimental/rust_png/...

gn / ninja

To build the code from this directory from Skia:

1. gn args out/RustPng and set skia_use_rust_png_decode = true as well as skia_use_rust_png_encode = true
2. gn gen out/RustPng
3. autoninja -C out/RustPng dm

To test the code (via `tests/SkPngRustDecoderTest.cpp` and
`tests/SkPngRustEncoderTest.cpp`):

$ out/RustPng/dm --src tests --nogpu
--match RustPngCodec
RustEncodePng

TODO(https://crbug.com/356875275): Add support for running older tests
(e.g. ones from `tests/CodecTest.cpp`) against `SkPngRustCodec`.

## Differences between `SkPngCodec` and `SkRustPngCodec`

* `SkPngCodec`:
    - No APNG support.
    - No CICP support.
* `SkPngRustCodec` differences - see
  https://issues.chromium.org/issues?q=parentid:362829876%2B

## Differences between `SkPngEncoder` and `SkPngRustEncoder`

* `SkPngRustEncoder` differences - see
  https://issues.chromium.org/issues?q=parentid:381140294%2B