C adoption of C++ ok_color.h reference implementation

Originally made by Björn Ottosson

Blog post: https://bottosson.github.io/posts/colorpicker/

Source: http://bottosson.github.io/misc/ok_color.h

usage:

#define OK_COLOR_IMPLEMENTATION
#include "ok_color.h"

notable changes:

• Introduced Linear_RGB Struct to make the type more explicit
• removed redundant calls to find_cusp