docs: Offscreen Canvas fix touch pick and 2 fingers interactions

[Arecsu]

• Picking doesn't work properly (sometimes at all) in touchscreens due to the lack of clientX and clientY values in the proxy event. Added those.

• Added a event.preventDefault() in touchEventHandler() because, for some unknown reason, mouse events get fired in touchscreens when doing a simple tap. Happens only at Offscreen Canvas. Tested on Chrome for Android and Firefox for Android. Same behavior both. Might be intended per specification? This is the best fix I found to handle it.

• 2 or more fingers don't get fully handled in OrbitControls, resulting in a black screen when trying to pinch-to-zoom, among other scenarios. It expects a pointerId with our custom Pointer Event proxy. To be specific, it gets called here:

three.js/examples/jsm/controls/OrbitControls.js

Lines 1069 to 1077 in beab9e8

	_getSecondPointerPosition( event ) {
	const pointerId = ( event.pointerId === this._pointers[ 0 ] ) ? this._pointers[ 1 ] : this._pointers[ 0 ];
	return this._pointerPositions[ pointerId ];
	}

Fixed that as well :)

Also, wanted to express how good it is to have a fully working example of Offscreen Canvas + OrbitControls. I've learned so much about workers and code structuring with it. Offscreen Canvas are desperately needed in a website I maintain to keep a javascript horizontal slider to move smoothly while loading and presenting a 3D object. Without this example, I don't think I would have been able to do it.

Thank you so much!