createCanvas compatible API

[TiborUdvari]

We should consider an interface similar to the p5.js way of doing things. It would reduce friction if we could take an existing WEBGL sketch and change it to a WEBXR AR or VR sketch by doing something like this:

function setup(){
  createCanvas(1000, 1000, AR);
}

instead of

function preload() {
  createARCanvas();
}

The current p5.js implementation for reference:

/**
 * @method createCanvas
 * @param  {Number} [width]
 * @param  {Number} [height]
 * @param  {HTMLCanvasElement} [canvas]
 * @return {p5.Renderer}
 */
p5.prototype.createCanvas = function(w, h, renderer, canvas) {

It would have to add a parameter to consider starting the sketch automatically as well: #224

[stalgiag]

Yeah this makes sense. The reason that this wasn't done initially is that modifying p5 prototypes directly is not recommended in the library authoring guidelines for p5. That said, I believe we had to do this elsewhere so it wouldn't be the most extreme departure.

[TiborUdvari]

Yes, I think generally it's a good idea, but in think in these cases it would make sense. I think people would expect and interface like this.

I'm adding this feature with #235

This keeps the createARCanvas type functionality still working, although there are now a few lines like
p5.instance._incrementPreload(); in the __createButton that are not needed if the preload method is used for initialisation.

A future refactor could/should be done to better differentiate the two.