Exported diagram size should take curved edges into account

[varkor]

See for instance this diagram, in which the exported <iframe> crops the curved edges at the sides. This is because diagram_size does not currently take edges into account.

quiver/src/ui.js

Lines 2471 to 2487 in 4a5f197

	diagram_size() {
	let [width, height] = [0, 0];
	// Compute the extrema of the diagram.
	const bounding_rect = this.quiver.bounding_rect();
	if (bounding_rect === null) {
	return Dimensions.zero();
	}
	const [[x_min, y_min], [x_max, y_max]] = bounding_rect;
	// Sum to compute width and height.
	for (let x = x_min; x <= x_max; ++x) {
	width += this.cell_size(this.cell_width, x);
	}
	for (let y = y_min; y <= y_max; ++y) {
	height += this.cell_size(this.cell_height, y);
	}
	return new Dimensions(width, height);
	}