Removing inject `_this` object.

[kalwalt]

In various part of the code we can found var _this = this because otherwise the this object is unvailable, because the use of nested functions example in ArToolkitContext in the init function:

AR.js-threejs/src/ArToolkitContext.ts

Line 138 in 9c8e08b

var _this = this;

and then after:

AR.js-threejs/src/ArToolkitContext.ts

Lines 144 to 153 in 9c8e08b

	function done() {
	// dispatch event
	_this.dispatchEvent({
	type: "initialized",
	});
	_this.initialized = true;
	onCompleted && onCompleted();
	}

maybe we can rewrite these functions?

[kalwalt]

maybe intead of using the _this variable trick we can use bind() ?