Rust PNG decoder

This directory contains experimental code for providing SkCodec API for decoding 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 use and test the code from this directory from Chromium:

1. Ensure that https://crrev.com/c/4723145 and https://crrev.com/c/5747366 have landed or have been cherry-picked to the local repo
2. gn args out/... and set enable_rust_png = true
3. autoninja -C out/... gfx_unittests
4. out/.../gfx_unittests --gtest_filter=RustEnabled*

Skia build instructions

TODO(https://crbug.com/356875275): Code in this directory is not yet covered by Skia standalone builds.