Add startDraw to Draw.Rectangle

[Jesperi02]

I needed to skip first click event of drawing a rectangle, because i wanted to start drawing from the maps right click event. I added a "startDraw" function to Draw.Rectangle, so i could do that. It's pretty much the _onMouseDown event of Draw.SimpleShape copy pasted. This works great on my machine, hope it work for others aswell.

Here is how i use it:

map.on('contextmenu', function(event) {
	let rectangle = new L.Draw.Rectangle(map, {
		shapeOptions: {
			color: '#3388ff',
			fillColor: '#3388ff',
			weight: 3,
			opacity: 1,
			fillOpacity: 0.2,
		},
		showArea: false
	});
        rectangle.enable();
	rectangle.startDraw(event.latlng);
});