Fix link positioning

[eKoopmans]

• scaleFactor (k) is now correctly applied when calculating link rectangle and textWithLink height
• Also removed redundant declaration of jsPDFAPI.link, which was declared twice

Closes #638
Related issues/PRs: #170, #390

[eKoopmans]

Here's a demo of the problem behaviour. Notice that in the first example, the link positions do not correspond with the text/rectangle locations.

Current behaviour: jsFiddle
Behaviour after PR: jsFiddle

[Flamenco]

Great fiddle, and thanks for the contribution! I will try to get these in over the next few days.

[eKoopmans]

Thanks!