Add gamut D library

[ryuukk]

https://github.com/AuburnSounds/gamut

Announcement: https://forum.dlang.org/post/rnnvumdfuldmmruhiyjo@forum.dlang.org

Supports:

• QOI: 8-bit, RGB/RGBA
• QOIX: 8-bit, L/LA/RGB/RGBA. This is an evolving format, specific to Gamut, that embeds some developments in the QOI family of formats.