Mono: Avoid dictionary lookup for common colors

[AbeniMatteo]

Accessing statically defined colors from Colors is now an O(1) operation.
-color is also affected since it is implemented as Colors.White - color.

Namespace	Method	Mean	Ratio	Code Size
Old	GetColorByProp	4,762.49 us	1.00	1,811 B
New	GetColorByProp	71.72 us	0.02	46 B
Old	InvertColor	2,066.48 us	1.00	1,602 B
New	InvertColor	265.14 us	0.13	184 B

public float GetColorByProp()
    var r = 0f;
    for (int i = 0; i < 100_000; i++)
        r += Colors.Red.R + Colors.Black.R + Colors.White.R;
    return r;
}

public float InvertColor()
{
    var color = Colors.Red;
    for (int i = 0; i < 100_000; i++)
        color = -color;
    return color.A;
}

[raulsntos]

Looks great, thanks a lot!

[RedworkDE]

Doesn't accidentally change the value of any color.

[YuriSizov]

Thanks! And congrats on your first merged Godot PR!