`readCanvasColor` returns empty Color object if text is outside canvas boundaries

[shivaprsd]

I didn't know this could happen in a normal PDF!

This eventually throws an exception in getTextStyle, when .deltaE() is called on the empty Color.

This causes shivaprsd/doqment#33 and shivaprsd/doqment#34.

[shivaprsd]

We need a check here, before trying to access the RGB:

doq/lib/engine.js

Lines 219 to 223 in 977ecf7

	let {x, y} = tfm.transformPoint({ x: tx + dx, y: ty - dy });
	[x, y] = [x, y].map(Math.round);
	const i = (y * canvasData.width + x) * 4;
	const rgb = Array.from(canvasData.data.slice(i, i + 3));