[Core] Make color type configurable (#75)

[matevzmihalic]

Allow users to define Clay_Color type which would allow us to use the same type in clay and in renderer which would omit the currently needed conversions.
I've also added CLAY_COLOR_RGB and CLAY_COLOR_RGBA that should be used instead of hard coded color structs.

To use clay with grayscale renderer you would need to define this:

#define CLAY_COLOR uint8_t
#define CLAY_COLOR_RGBA(r, g, b, a) (uint8_t)roundf(0.2126 * (float)r + 0.7152 * (float)g + 0.0722 * (float)b)

If you think this is an ok solution we also need to change two more things in debug view:

• changing color for highlighted row
• outputing color string (currently in {r: 0, g: 0, b: 0 a: 0} format)

[nicbarker]

Thanks for opening this one, I will take a look today or tomorrow 🙂

[nicbarker]

We should also do this for Clay_String I think, most users I suspect will have their own string container struct and it would be nice to avoid duplication boilerplate