Fix annotation layer misalignment issue - #256

It's probably not going to be merged as the owner doesn't seem to be active on this package.

However, there is no point to multiply by 1, if window.devicePixelRatio is creating problems you might just remove the multiplication.

so instead of * 1 you just write:

var scale = canvasElt.offsetWidth / pdfPage.getViewport({ scale: 1 }).width

Thanks for your input. I did try this originally and it didn't always fix the issue in some edge cases. 1 is the second condition, it multiplies by window.devicePixelRatio if the relevant condition is met.