Add color helper functions to dang-math.

[Possseidon]

Add color helper functions to dang-math.

All in namespace dang::math::color.

Utility

• clamp
• gray
• rainbow
• color constants

Conversions

There are various representations that should be convertible to each other:

• rgb (byte and floating point)
• hsv (byte and floating point)
• hex (std::uint32_t)