Summary: Struct RGB/RGBA/etc. for sharing pixels between crates

Description:
`struct RGB/RGBA/etc.` for sharing pixels between crates + convenience
methods for color manipulation. Allows no-copy high-level
interoperability. Also adds common convenience methods and implements
standard Rust traits to make `RGB`/`RGBA` pixels and slices first-class
Rust objects.

URL: https://crates.io/crates/rgb
License: MIT

Maintainer: tv