Add atan signature with two input

[nkint]

To be a little bit more compliant with GLSL spec atan function has two signatures.
See here: https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/atan.xhtml

[postspectacular]

Thank you, @nkint - I was aware of the two arities, but settled for the 1-arg version so far, since the other sig would also need special treatment in the JS code gen and I was hoping to work on this as part of #96. So I'd propose we do support the 2 arities of the atan builtin, but emit only the single arg version (by first dividing a over b, if needed), at least for now:

export function atan<T extends Prim>(a: Term<T>): FnCall<T>;
export function atan<A extends Prim, B extends A>(a: Term<A>, b: Term<B>): FnCall<A>;
export function atan(a:Term<any>, b?: Term<any>) {
    return b
        ? builtinCall("atan", a.type, div(a, b))
        : builtinCall("atan", a.type, a);
}

[postspectacular]

Actually, since the 2 atan versions have different output ranges [-π/2, +π/2] (single arg) vs [-π, +π] (2 args), it should be like this:

export function atan<T extends Prim>(a: Term<T>): FnCall<T>;
// prettier-ignore
export function atan<A extends Prim, B extends A>(a: Term<A>, b: Term<B>): FnCall<A>;
export function atan(a:Term<any>, b?: Term<any>) {
    return b
        ? mul(builtinCall("atan", a.type, div(a, b)), 2)
        : builtinCall("atan", a.type, a);
}

(also adding prettier-ignore to keep fn overloads more easy on the eye)

[postspectacular]

This will make it temporarily minutely slower, but at least is correct... and once the function dispatch updates (#96) are done, we can update this here once more

[postspectacular]

Soo... scrap my 2 last comments, the differences are (sadly) quite different. See this little comparison I just made: http://glslsandbox.com/e#56557.0

So, let's just keep your version and I will add a note to the readmes that atan w/ 2 args is not supported by the JS code gen yet...

[postspectacular]

Also just once more for reference (note to future self): https://en.wikipedia.org/wiki/Atan2#Definition_and_computation