Added new Color operations

[ggetz]

Added new functions to Color, including:

• add - Adds two colors componentwise
• subtract - Subtracts two colors componentwise
• multiply - Multiplies two colors componentwise
• divide - Divide two colors componentwise
• mod - Mod two colors componentwise
• multiplyByScalar - Multiply a color by a scalar componentwise
• divideByScalar - Divide a color by a scalar componentwise

And updated ColorSpec for each of these new functions

[emackey]

I don't normally hear of divide or mod in regards to color. Is there a use case in mind for color division?

More commonly, the opposite (or negative) of a multiply type color operation is called screen or some such, and the formula looks like this:

vec3 result_color = 1.0 - ( (1.0 - color_A) * (1.0 - color_B) );

This is kind of the photo-negative effect of multiply, and is useful for superimposing brighter things on darker things etc.

Also, diff is sometimes useful, it's essentially an absolute-value version of subtract.

Just my $0.02, not saying anyone needs to change the design here, just something to consider.

[pjcozzi]

@emackey this is for the 3D Tiles styling language where Color is a first-class type just like Number, String, etc. I think it is important that we implement all the sane operators for completeness even if they wouldn't be widely used; think of this like a color that is a vec4 in GLSL.

As for interesting color ideas, feel free post them here: CesiumGS/3d-tiles#2

[pjcozzi]

P.S. it would be a killer code sprint project to extend 3D Tiles styling with new color operations...gamma correction...lighten...darken...whatever. We could only put the pragmatic ones in the spec though.

[pjcozzi]

@ggetz code looks good. Update CHANGES.md.

[ggetz]

Updated.

[pjcozzi]

Thanks @ggetz!