Re-render follow hint overlays on view resize

[aidanholm]

It's slightly annoying to resize the browser window (or, if you're using vertical tabs, to open a background tab from ex-follow mode) and have all the hint overlays shift to the left or right. It shouldn't be necessary to rate-limit this re-rendering, because the slow part of displaying follow hints is enumerating and filtering the candidate elements, and re-rendering need only redisplay the already-filtered element set.