When the same text is highlighted multiple times in a PDF, it could be hard to
distinguish focused highlights as the `<rect>`s for focused highlights
could be obscured by `<rect>`s for overlapping unfocused highlights.

SVG elements are painted in the order they appear in the document and don't
yet support a z-index property [1], so this commit fixes the issue by moving
`<rect>`s to be last amongst siblings when focused.

[1] The SVG 2 spec does support a z-index attribute, but none of the major
    browsers support it yet. See https://bugs.webkit.org/show_bug.cgi?id=90738,
    https://bugs.chromium.org/p/chromium/issues/detail?id=670177 and
    https://bugzilla.mozilla.org/show_bug.cgi?id=360148.