libimagequant bindings for Rust

Imagequant library converts RGBA images to 8-bit indexed images with palette, including alpha component. It's ideal for generating tiny PNG images (although image I/O isn't handled by the library itself).

This wrapper makes the library usable from Rust.

Rust API closely follows the C API, but is slightly OO-ified:

liq_set_dithering_level(result, 1.0);
  ↓
result.set_dithering_level(1.0);

For more details see libimagequant documentation and Rust function reference.