Make the code more CSP friendly

[tdelmas]

The current version use the Function constructor which make the code incompatible with a secure CSP.

https://github.com/stackgl/gl-shader/blob/master/lib/create-uniforms.js#L9 :
For example:

//Binds a function and returns a value
function identity(x) {
  var c = new Function('y', 'return function(){return y}')
  return c(x)
}

Could probably be replaced by something like:

//Binds a function and returns a value
function identity(x) {
  return function (){return x}
}

The two only other functions using Function are makeGetter and makeSetter.

Related: plotly/plotly.js#897

(sorry @mikolalysenko I feel like I spam you but it's just a coincidence if it's again one of your repository! And I think it's important to document which code can or cannot be used in a secure manner in a browser)