When scrolling to an anchor in a page that has not been rendered by
PDF.js, the anchor actually references a placeholder element in the
middle of the page. In order to scroll to the correct location in the
page scrolling for such anchors needs to happen in three phases:

 1. Scroll to the approximate location of the final anchor, given by the
    placeholder anchor. This will trigger PDF.js to re-render the target
    page.
 2. Wait for PDF.js to finish re-rendering the page and for the client
    to finish re-anchoring the annotation.
 3. Scroll to the real/non-placeholder anchor.

Change `PDFIntegration#scrollToAnchor` to implement the above steps.

Fixes https://github.com/hypothesis/client/issues/3269