Search For Canvases Under User-Supplied DOM Node

[twavv]

This is causing some issues in a project where we use IFrames and share some libraries between them (explicitly, JuliaGizmos/WebIO.jl#225 is where I lay out the issues we're facing).

Essentially, I'd like to change this line

p5.js/src/core/main.js

Line 342 in edb41cf

var canvases = document.getElementsByTagName('canvas');

to be something more like

    var dom = this._userNode || document;
    var canvases = dom.getElementsByTagName('canvas');

I can open up a PR if you'd accept it but I wanted to hear back first.

[meiamsome]

I think there's definitely some merit to such a change - although there are plenty of instances where we're using document elsewhere in the library that might also need similar checking?

This seems helpful in a general case too because it would prevent us walking over all canvases on the page that don't belong to us and hiding them all. (Use case of multiple p5 sketches, or just being included alongside other canvas elements)

In particular, perhaps after this part of the code:

p5.js/src/core/main.js

Lines 235 to 240 in edb41cf

	if (this._userNode) {
	if (typeof this._userNode === 'string') {
	this._userNode = document.getElementById(this._userNode);
	}
	}

I think we could add

    this._userNode = this._userNode || document;

and then use this._userNode more often throughout the code-base.

[Spongman]

could we use the p5.prototype._elements array for this?