This patch implements support for line annotations. Other viewers only
show the popup annotation when hovering over the line, which may have
any orientation. To make this possible, we render an invisible line (SVG
element) over the line on the canvas that acts as the trigger for the
popup annotation. This invisible line has the same starting coordinates,
ending coordinates and width of the line on the canvas.
Even though the PDF specification does not state that `Opt` fields are
inheritable, in practice there are PDF generators that let annotations
inherit the options from a parent.
Every other unit-test in `annotation_spec.js` is already only testing the annotation code. Hence it seems unnecessarily convoluted to make use of the API here, when we can (fairly) simply provide the necessary data explicitly as in all the other annotation unit-test.
